Polyester finishing techniques
Currently, I spray 3 - 4 coats of polyester with 15 minute intervals, as per their technology, then dry for at least 48 hours. The material right out of the can is very dense and demands to be thinned with purified acetone. However, the more you thin it, the less it is able to get coated with a layer of paraffin when dry - there is a fine line. I have to keep it dense enough and yet be able to spray it. Quality of spray exhibits some orange peel and cannot be polished with compound directly. Rather, I must sand, starting with 400 and ending with 1600, which is very labor intensive.
The problem with sanding is that it is difficult to stay within a single spray coat--in some places you go through to a second coat, exposing the boundaries. After that, when I follow with a series of compounds, I see swirl marks. It turns out that once you touch the surface with sandpaper, it is virtually impossible to remove swirl marks, no matter how careful you are or how much time you dedicate, especially when you deal with large surfaces.
I would forget the paraffinated poly and go with direct gloss poly or polyurethane instead. Which ICA product are you using?
I use all automotive products for rubbing and polishing. They seem to have a product for every purpose.
From the original questioner:
I like paraffinated stuff. Even with non-paraffinated, no matter how you spray it, if your goal is to achieve piano look, you're gonna have to rub it. I am not just looking for shine with no swirl marks - this is simple. I am looking to get a mirror flat surface. I have seen this on some finished products, so I know it is possible. I am fine doing away with sanding, but so far I was unable to spray it with satisfactory quality in order to start with compounds directly. I am dealing with black polyester, which is much more revealing in terms of any mistakes.
I have produced flawless finishes with the system I use. The product I use is a polyester sealer (ILVA) to get my fill. Then I start sanding with 220, working up to 400, then applying a two part urethane (desired sheen). Depending on how clean your spray room (system) is, this could be enough. If not, the manufacturer makes a two step compound/polish to use.
I did a conference table several years ago, over 16' long, using this system. The customer thought it looked nicer than his brand new black Jag!
If you were to look at my finished product you would not know it was 90% polyester and 10% urethane. The urethane is very durable, but what's nice is it's high in solids, but it lays on the surface with ease. Also, the compounds are made for these materials specifically, so they work hand in hand.
From the original questioner:
I am a strong believer in polyester. It does not require any filler on any kind of surface, because it acts as a filler itself. I spray first two coats and then sand to 400, effectively using it as a filler. Then spray the rest of the 2 - 4 coats with 15 minutes intervals, waiting for paraffin to show up. Currently I use 3m sandpaper and rubbing compounds.
Urethane is not as hard as polyester. Hardness as well as thickness of the finish is very important to me. Urethanes can not be built as thick as polyester can. I have gone through the entire line of Mohawk finishing products including their ultra-catalyzed clear coats. They are just not what I am looking for.
My production is high-end speaker cabinets. When you do a table, you are dealing with a single surface with no joints. Speaker cabinets have many joints. The finish that I use must be able to fill and hide the joints and withstand the test of time, not sinking into the joints.
I do understand where you're coming from--the polyester is your build, lay it on as heavy as you like. The 2 pt urethane that I use is as hard as rocks, and remember, the urethane is just used for your final coat. Many use a two step system such as this. Mohawk has its market and it's not in quality European coatings. And if you do want an absolutely flawless finish, invest in a water filtration spray booth and eliminate the need for compounds and polishes. Again, once you achieve the build you need, and your surface is flat, why not go with a quality coating--high gloss and high solids and extremely durable--for that final coat and minimize, if not eliminate, all your compounding labor.
Mohawk DOES NOT have a two-component urethane. They have a urethane fortified pre-cat lacquer, but this is about as far away from a true two-part urethane as a Chevy is from a Cadillac. The solution to your need to excessively thin the polyester is an El Cheapo gravity feed gun from Asturo. Get a 2.5 mm $75 gun from IC&S and hose the polyester on at nearly full viscosity. Flows like butter. The gun looks ugly but the results don't. Blowing polyester with a turbine is sort of like trying to eat a Wendy's Frosty through a straw.
From the original questioner:
Tonight, spending another few hours in the shop I did manage to buff the polyester surface to near perfection. After wet-sanding 320, 400, 600, 1000 grit, I followed with compounds from PRESTA:
Strata 800 (removes 800 grit sand scratches)--I applied this compound with a 12" random orbital polisher equipped with genuine lamb's wool bonnet. I managed to get rid of all sanding scratches. They then recommend using their Chroma 2000 and then Aurora 3000. However, I did not have those and finished with my 3M hand glazing compound. That was the surface I was looking for. It could be scratched with a soft cheese cloth! I believe I need to get their finer compounds and try them also.
I should get a gravity feed as suggested and dump 3 - 4 thick layers for polyester. After that, the surface will obviously be rough with orange peel. However, the thick last layer should allow to cut it smooth with sandpaper (320 - 1000). Then I will follow with my compounds.
It would be ideal I could find a way to dump polyester without thinning and get good atomization and a super flat surface. My spray booth, however, does not have water filtration. I should try that spray gun. After screwing with the Fuji system for two years, I don't particularly like it.
From the original questioner:
After I get this solved, I have an even more interesting task: applying original (OEM) Porsche black lacquer with metallic dust over MDF (another speaker). I guess I will use polyester as a sealer.
You need to reevaluate your whole process here. I understand that you want that flat mirror finish, but you need to change a few things. For one, go to a direct gloss polyester rather then the pariffinated. Keep your room extremely clean for trash purposes. Use a polyester primer under your topcoat for leveling and filling properties--it is difficult to do what you are doing with topcoat alone. Get rid of any HVLP equipment for spraying polyester. Conventional air spray guns or air-assisted airless or airless guns work best.
You may also want to look at reducing the amount of accelerator in the polyester if you are showing your coats underneath. When polyester is properly applied, you will have just one extremely thick coat with no sign of the layers underneath. You can use your primer to level and fill, then grind it down with P150-P220 sandpaper. Then topcoat with the direct gloss polyester. Direct gloss lays down better then pariffinated. Polyester as a rule generally will not lay down smooth, but you should see a big difference between the pariffinated and direct gloss.
Your buffing schedule also needs improvement. With polyester, you want to level first with an aggressive paper, then the rest of the sanding is just to remove scratches. Starting out with a lower grit paper, then stepping up in progression to 1500 grit. Depending on how rough the polyester is before sanding will determine the grit required to start your sanding schedule. It is not unheard of to start sanding polyester with a P220 grit, then move up in progression to 1500. This may sound strange and time-consuming, but you level with the aggressive paper and just remove scratches until you reach the 1500 grit.
I have sprayed some huge conference tables with polyester and buffed them out to a flawless high-gloss. Sadolin's chemist, who formulates their polyester, came to our shop and taught us how to spray their product. The first thing is to spray on a wet coat and let it set until you get a little "stringing" when you touch it with your finger. That's when you spray on the next coat. I think you are waiting too long between coats and that is giving you your "halo effect." If you spray properly, this should not occur.
We also used 3M microfinishing film to do our sanding. It is available in micron grits. I used 20 micron, then 15 micron and finally 9 micron--this is available in a 6" diameter adhesive disk to use on a Dynabrade sander with a NEW, FLAT pad. Wet sand, using water.
You will not need to do as much buffing with this system. Finally, and probably most important, is your buffing compound. The VERY best I ever found was Sadolin Canada's Pasta Abrasiva, which I am no longer able to find. I tried many different compounds with extreme frustration, for as you know polyester is very difficult to buff if you do not have the proper compound. I too have used a catalyzed automotive urethane over the polyester in some situations, mainly due to the ease of buffing. Also, I used a Binks 2 quart plural component gun to spray the polyester, which made life much easier.
From the original questioner:
I got in touch with ICA, and they made many of the same valuable observations as you do, like to insure that you get stringing of the polyester coat as an indication of when to spray the next coat. I have to admit that strict adherence to spraying procedure is *absolutely* critical. I made some deviations off that procedure, which contributed to the unsuccessful results.
I find especially valuable your recommendation of 3M microfinishing film and Dynabrade sander. That is something I need to try immediately. With the process I am currently using, this may be the last link to success. Manually I am unable to achieve absolute uniformity and consistency of the surface grit. Often I finish with grit 1000, but in reality there are some deeper grooves which you absolutely can not see, until you start polishing. If you go back and try to eliminate them, you will scratch the surface in other places. Using 3M microfinishing films along with the machine may solve that significant problem.
The quality of the surface I am looking for exceeds car finishes. I am looking to get absolutely uniform tiny polyester grain without the surface looking like it's been polished to hide imperfections. I can easily get it to deep high gloss and black mirror now--however, not for the critical eye.
Speaking of component spray systems, can paraffinated polyester be applied successfully with component spray? So far, I see three obstacles:
1. Catalyst and accelerator must not be mixed directly with each other. Even though they are mixed in the air stream, is it still safe? Mixing those two components directly results in lots of heat.
2. Mixing catalyst with base and accelerator, causes normal chemical reaction between them during which the mix must dwell for 5 minutes. In that time lots of air bubbles appear and escape the mix. If mix is not allowed to stand for 5 minutes and sprayed directly after (or during) mixing the components, those air bubbles will form on the surface being sprayed.
3. There is no component spray system I've heard of which is capable of providing mix ratio of 1:50 which is required for polyester formulae (2% catalyst and 2% accelerator). No acetone should be added if possible, as it degrades quality of the finish and may impair migration of paraffin between layers.
What about it?
You are adding too much cobalt to your polyester, which is creating the bubbles you see. The 2% is a starting point--you can drop the amount of cobalt you are adding to the polyester. The cobalt helps to cure the polyester faster or slower. You need some cobalt, but not necessarily 2%. MEKP must be added at 2% for proper cure, but not your cobalt. Also, the acetone doesn't weaken the polyester--it helps improve the flow from the gun.
Get a gram scale to measure the very small amounts, like 2-3%. You can be very accurate on the scale. Also, I hope you are using 100% virgin acetone (ICA D1016)--this will help also.
From the original questioner:
I was just following ICA recommendations imprinted on the catalyst and accelerator, which state to mix 2%. I will try to play with the amount.
I use purified acetone supplied by ICA.
A rep from ICA suggested that excessive thinning with acetone (beyond 5%) impairs the ability of paraffin to migrate to the top layer.
On my previous post about component spraying: I think it is possible to reduce the number of components and mix ratio to manageable limits. I think I can premix accelerator with base and catalyst with base. Mixed separately, they will not cure. They will only cure once mixed in the airstream in 1:1 ratio.
I just had a crazy idea: since I don't have access to a component spray system, why not attempt using two guns and arrange their streams to intermingle? Will this work?
Binks makes a 2 quart plural component pressure pot system that would be perfect for you. You will have to experiment with the catalyst ratio. You need more catalyst in these systems because you do not have the volume of material you do when you add catalyst to a whole pot of material, therefore not as much heat is generated. I don't like your chances with spraying two guns together, but I would like to watch.
On mixing the polyester like you mentioned, you can bring both components in and mix at a one to one ratio. For example, if you need to spray 5 gallons of polyester at a time, mix 2.5 gallons with 4% MEKP and the other 2.5 gallons with 4% of less cobalt. The two won't harden until they are mixed one to one, with the exception of the 2.5 gallons mixed with the MEKP, as it will harden without cobalt in about 8 hours, and that time depends on the temperature. Reducing your cobalt will reduce your cure time and also keep it from foaming up and trapping air. It is standard for polyester to be mixed 2% MEKP and 2% cobalt, but you can play with the cobalt to improve the workability of the polyester, but not the MEKP. You also do not want to exceed 2% MEKP, as this will make the polyester brittle.
You are probably correct on the acetone in the pariffinated polyester. I have yet to work with pariffinated polyester and know that it is sensitive to temperature changes and important for the pariffin to move to the top. They may have a polyester reducer that dries slower then acetone that could help you on thinning the material. We don't use ICA and I know that different paint companies have different specs, but the technology over one manufacturer to the other is basically the same, as there aren't any trade secrets in some areas. I do think you should consider a direct gloss and polyester primer underneath. There is a reason for this that I won't mention here.
The above is correct--when mixing polyester, always use 2% catalyst and take advantage of the ability to control your cure time with the cobalt by regulating from as low as .5% to 2%. Also, if bubbles and wax coming to the top of your film continues to be a problem, take a hot gun and blow some hot air onto the film to help melt the wax and allow its migration to the top. (This is just a trick you may want to play with.) Paraffin polyester can be tricky in different weather conditions, and there are different melting (at temperatures) waxes available also. When mixed properly, polyester can save a lot of time when attempting to achieve filled finishes.
From the original questioner:
I think sanding and rubbing must be done to achieve a flat surface, and everything I have looked at had sanding marks, including pianos and high-end speaker systems. They are difficult to notice, unless you know what you are looking for. I think once I get rid of hand sanding and finally try 3m microfinishing film with a machine, I should get it solved.
Joinery is a whole separate subject. I spent around 2 years until I found a simple way to join MDF panels, so that the joint does not show up on the finish. You just have to do a long 45-degree miter joint and glue the panels together. This way, glue penetrates all MDF layers--the joint is very strong and stable. It will not move and crack the finish. Of course, you do not always deal with 90 degree corners and square panels. This method of joinery becomes quite an exercise then.
Component spray is interesting. Unfortunately, one has to spend a lot of money to get a decent BINKS component airless air-assist setup. Otherwise, a component HVLP pressure pot can be had for less. Despite the fact that I have learned how to lay polyester at full viscosity with my present HVLP gun, short pot life is still highly inconvenient.
Have you tried lock miter joints on your MDF corners?
From the original questioner:
MDF will split in layers if you exert even the smallest force across the joint. The best thing you can do is cut it 45 degrees and glue the entire joint surface area. Also, I often have to join the panels at angles other than 90 degrees. These angles make it challenging cutting any joint profiles with MDF. Oftentimes, profiles will be brittle because of MDF layer orientation.
As to my polyester status, I had another conversation with ICA. I am getting their polishing compounds to see how they work compared to the stuff I used. They also suggested importing some European carbide sandpaper.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
Comment from contributor C:
I build high end shadowboxes, display cases and flag cases. I originally came from the corporate aviation cabinetmaking industry, and their finishes are exquisite. Several different firms I worked for used varying methods to achieve the ultimate mirror finish, but I saw problems they had and wondered what could be done to simplify them. Of course, these companies had the financial means to invest in costly finishing booths/clean rooms and drying equipment, whereas I do not.
The most recent company I worked for used a UV formula, and it looked very nice, but their problem (though they were more than ready to blame the cabinetmakers for improperly applying the adhesive to the veneer) was that the ultraviolet lamps they use heated the object more than enough to cure the film, but caused delamination of the glue as a result.
Another firm used polyester, and yet another used urethane. Because of the manpower they each had, they were, of course, able to produce outstanding results, but I'll bet if you were to examine their bottom line, they were losing money like a wild man. 15-20 people in the sanding room, either sanding or re-sanding (and many times, burning through the previous coat), 3-4 people in the "touch up" room fixing the screwups, 5-8 people in the spray room, and then 10 or so people in the polishing room, and three shifts and countless hours involved.
I was in the cabinetshop, and therefore, now that I have my own business and am attempting to achieve the same type of finish without all the hassles, I am looking for the best finish to apply without all those calamities.
I manufacture my products with varying wood species, some exotic. Obviously, some species have more open grain than others, and as I am using a very generic Minwax sanding sealer (looking for ideas here...) I think I am losing the battle from the get go. I am losing faith in the Minwax line as more "Harry Homeowner," since the results are less than expected. I educating myself about using powdered stains, better quality sealers, etc. Something I read recently led me to believe that there is some sort of spread one puts over the entire substrate, much like a thin viscous putty, and then lightly sands off, that is colored according to the species one is using (maybe one needs to stain this putty to suit).
After this product (the sealer, applied with 1-2 coats only) has dried, I Scotchbright this layer, clean it with Prep-Sol and then apply successive coats of Dupont ChromaClear Urethane #7500S. Since this is a catalyzed finish, each coat to coat and a half takes up one full day in my limited shop. And since I cannot assure a totally clean room (but do the best I can), I see some specks of debris here and there with each coat (creating even more work!).
I have to reduce the psi of my siphon gun (should look into a gravity gun) to about 20-25 psi. Therefore, I am required to sand the next three coats with 320 grit, and after the final coat I use 3M microfinishing paper, starting with 30 micron, then 15 micron and then 9 micron respectively, followed by rubbing and then polishing compound. Then and only then do I achieve the flat mirror finish I am looking for. But I am killing myself, adding way more hours than I can ever charge out to the job.
I am looking for better methods and chemicals to use to eliminate all this redundancy. My business is new, so I don't have the funding necessary to purchase a fancy spray booth, etc, but maybe I ought to look into polyester rather than urethane. You all have said that poly is harder than urethane. This would definitely help me out, since even after everything is finished and ready to ship out, the finish can scratch just by looking at it! You wouldn't believe the stress I incur while wrapping the items in 1/8" polystyrene foam and putting them into the shipping container. I thought about using Imron Clear, but I really don't want to play into Dupont's name game much more than I am. I am familiar with Sikkens but haven't personally used them. I will check out the few others mentioned. Getting a scale to measure amounts might be worth the effort, too.
To the person that originated this thread, building high end speaker cabinets, I commend you. You are really trying to achieve the ultimate gloss mirror finish. I wish you could see some of the furniture we built in these 40 million dollar jets. It would give you goosebumps! Just the galley (kitchen cabinet cluster) is $150,000 to buy finished (not including installation). Everything from start to finish had to be perfect, or they would bust your ass. I was not in the finish department, so I am in the learning curve here. But what I do know is construction (of course we used honeycomb materials, veneer and really strange fasteners to achieve this look, but you wouldn't be able to tell this).
I made an oath to myself when I started this business that I would never succumb to using pressboard or MDF to build anything but templates. I see this as cardboard. It is cheap, it explodes when it contacts moisture, it shatters when the least amount of stress is applied against it, and it weighs a ton. I could never use this stuff in my frames, for they would literally tear off the wall, and nearly double the shipping costs.
There is an old addage... "friends don't let friends buy MDF." You are building what I think are real quality speaker boxes, but why are you cheating yourself using MDF? You could go with Multi-Lam cabinet grade plywood, pre-veneered or not, in most any thickness, half the weight and double the strength, and the end line customer would know that his builder put the best quality ingredients into his product. I abhor MDF, particleboard or any other similar strata. Multi-lam plywood I accept. It stays straight as an arrow and costs the same (with the right vendor) or less. No MDF ever again in my house, unless it's for templates. You should see the Multi-Lam they have in Europe. 18 layers thick, striated in both directions all the way through, even in the 1/2" thickness. Hard as a rock, yet a 12' section is as straight as a ruler!
Comment from contributor K:
I build custom sportfishing yachts and work in the cabinetry end and run the whole finishing process. We do mainly teak and some cherry interiors. I always start with a spar varnish to seal my grain and give color. This step would be irrelevent to you, though. Next, I use Awl-Brite Plus from US Paints Awl-Grip division. I apply 8-10 coats of this. This is a 3 part clear urethane finish I use to build the grain in. It is very forgiving and can be applied every 3-4 hours with a max of 3 coats a day. You do not need to sand in between coats as long as you don't let it sit for over 24 hours. This gives good adhesion. When all the grain looks to be filled and I am about 8-10 mil thick, I sand with 220, 320, then 400. I sand as much as I can with my 5" Bosch random orbital at 220 and 320 levels. Then I hand block with a hard block at 320 and 400. I then shoot 2-3 more nice even coats of Awl-Brite. I let this dry for a few days, then start sanding with the micro mesh system. This system starts with 1500, 2400, 3200, 3600, 4000, 6000, 8000, 12000 grit. At this point you will achieve the most beatiful finish with a mirror shine even to the trained eye. Better than a piano.
Comment from contributor E:
We manufacture wooden interiors for cars such as dashboards, steering wheels and etc. To get the deep high gloss finish we put 2-3 layers of varnish on before applying the polyester layers, and that acts as sealer and gives more shine and depth.
Comment from contributor B:
Iíve been doing still-water polyester gloss finishes for about 7 years now. Iíve heard all kinds of tricks and tips and Iíve tried quite a few things. Nothing looks as good as still-water high gloss when itís done right and nothing looks as bad when it is done wrong. Iím in S. Florida and the market for high-end gloss finish I imagine, is about as good as anywhere else in the country. Iíve seen a lot of it, Iíve done a lot of it. I can honestly say Iíve never seen a reflection as good as mine (the gloss that is) except for some piano finishes.
A clear reflection with no distortions starts with your purchasing agent. Despite some of the criticisms of MDF, I prefer to use a high quality MDF core to anything else that Iíve tried. I suppose there are probably some plywood cores that are good enough to use I just havenít seen them. If there are any voids whatsoever in the core, even a 1/16 íí, theyíll be apparent in your finished product if you look for them. Donít underestimate the substrate factor, if you do, it doesnít matter what you do after that, you wonít have a perfect finish.
The next step is to figure out how the product needs to be built. If you screw or staple into the core, youíll see that too. I donít mean screwing or stapling into the finished surface, I mean into the underside of the substrate. A slight compression of the core occurs when you penetrate it with a screw or staple and if you polish it enough to get that mirror reflection, and that compression will telegraph onto your film. I suggest a glue block be applied if construction allows, and then you can screw into that.
The first thing I do when getting started is to apply a barrier coat to the reverse side of the substrate. If it is a tabletop that is getting finished, then you need to apply the barrier coat to the underside. If it is a door, then apply it to the back of the door. If it is a door, itíll be important to put the same quantity of finish to the backside as the front to offset the effects of the thick film. If you miss this step, the tension of the film will literally warp your product. The importance of this barrier coat is to lock in and stabilize the wood. Again, if you miss this step, youíll be unable to achieve the still-water look and youíll notice the grain of your wood slightly telegraph through the film. Most people donít even notice this, but if it is polished correctly and you have a discerning eye, itíll be apparent. This defect is the kind of thing you have to almost train your eyes to see like one of those cross-eyed 3D puzzles, but once you see them, you canít see anything but them. A coat or two of finish will not due the trick, you need to create an absolute barrier. In South Florida most of the shops run without air conditioning and our high humidity causes the wood to swell ever so slightly. Once the job is installed in a customerís air conditioned home, that humidity will dry out and thatís when youíll see the defect.
Iíve been using ICS ILVA polyester, specifically the TG1323. I start with a 50/50 coat of TF25 Isolante and I apply as many coats as needed until the surface stops ďdrinkingĒ the finish. The thinning allows the finish to penetrate deeply into the veneer and lock it in. After it stops drinking, I apply it reduced at about 10%. Until I get a decent penetration of any open grain. This stuff dries as hard as glass (or thereabouts) and I feel the more you put on (within reason), the less trouble youíll have later. I like to get it to the point that the grain is no longer open enough to release an air bubble. It need not be grain filled, but enough that it has coated the inner surface of the actual grain. Again the reason being, polyester tends to flow over grain because of its viscosity, therefore air bubbles tend to be trapped in the grain and can cause a whitish spot within the grain. If time allows, I prefer to let this dry at least a day before moving on.
The next step is to sand the isolante. Before applying poly, you need to blow out all the grain with an air nozzle set to no more than 50lbs of air pressure. Anything above 50lbs can cause a static charge on your surface. In my experience, static charges that attract dust on your next film application are more of a problem on polyester or urethane than lacquer. There are also filters you can hook up to your air line that will eliminate the static charge. If need be, you can also wipe down the isolante with acetone so long as you donít let a puddle build up, in fact, keep your lint free rag only as saturated as need be to wipe down the surface. If you are working with latex gloves, be sure they are the un-powdered type as well.
The next step is the application of your polyester build coat. This is where I use the TG1323. Depending on your environment, you may have to adjust the mixture from 1-2% accelerator. I like to flood the surface with very low air pressure to get better penetration of the grain. Obviously a higher air pressure will get better atomization, but it can also in some circumstances, prevent the resin from penetrating the grain due to reflection of the product from the higher air pressure. To my way of doing things, a lot of sanding is inevitable so Iím not too concerned about having a perfect film application. If you want still-water, you have to sand and polish a lot. If you want to get it done quick and donít care about a slightly distorted reflection, then by all means crank up the air pressure. Two-part component spray equipment is ideal, but barring that, I just use an HVLP gravity cup gun. I have a guy doing nothing but mixing as this stuff kicks in 10 minutes down here and Iíll apply quit a bit (Iím not sure in mils) in 10 minutes. Of course it all depends on your application, when I say I apply a lot, this assumes a flat/horizontal surface.
Obviously a vertical surface is a different story. Very rarely will one application do the trick (for me) and the more you apply, the deeper your reflection will be. The Ilva product doesnít have too much of a clarity issue, Iíve seen some other Italian stuff that does so youíll have to keep that in mind. The Ilva poly does have a green cast to it so if you plan to have a deep reflection, youíll need to adjust your stain/color a bit lighter and a bit redder to compensate. If I need to apply more poly, Iíll generally give it about 30 minutes to setup and then apply a second coat, maybe even a third. At this point, the spray environment is crucial. A positive pressure booth is ideal, but excluding that, you need to have a very clean work area including yourself-a bead of sweat on your brow can cost you a lot of time. Damp newspaper on the floor will help out.
The next step is sanding your build film. Most major imperfections can be handled with a fine drip file. Be careful how you sand out an imperfection. As you are going after that imperfection, you are also treating the surface around it and the tiny bit of extra attention to that area can show up in the finished product. I have no problem using 220 on a 6Ē DA. Make sure your pad is new and feels right. Some thick pads tend to grab and skip across the surface, I use a thin pad. Let the sander do the work, donít get impatient and start bearing down on it. Keep the tool running just fast enough to keep a consistent glide across the surface - do not crank the air pressure on the tool all the way up. Keep the work area clean, a spec of something coarser than the 220 will cause you a lot of problems. You must treat the entire surface the same. Be methodical and use long strokes in one direction, than run counter to that tracing out an ďXĒ and the go diagonally across and over and over again until you feel your surface is flat. Iíve also used blocks, but I prefer to use the sander for time reasons and maybe block it for the final sanding. It sounds silly, but youíd be surprised - hold the sander flat! Donít get aggressive sanding at angles.
This is the tedious part of the work, but it makes all the difference. What appears to you as flat can be something very different once polished. Once I think Iím finished, Iíll sand it up to 320/400. Then Iíll take 220 and lightly go over the surface applying just enough pressure to keep the tool flat. At this point clean your surface and observe the scratch mark patterns. If you are flat, there shouldnít be any areas that are shiny (relatively speaking). If you see areas that are shiny (from the 320 sanding), then you arenít flat. Donít sand the area where the shiny spot is, sand the entire surface again and repeat until the surface is free of the shiny patches. It is best to let your poly setup for 2 weeks before sanding if possible. This will allow the substrate and the build coat time to setup so there is minimal movement/shrinkage. If you sand the next day after application, and then perform the 320-check in a few weeks, youíll notice it is no longer perfectly flat-it moves/shrinks.
Next step, I apply my gloss coat. I prefer to use a polyester gloss coat, itís harder to sand and buff than urethanes, but the end result is a lot better in my opinion. Pretty much the same principles apply to spraying the gloss coat as the build with the exception of the lower air pressure. At this time you shouldnít be filling anything and the layout quality of the film is the most important. You also donít need as many coats, just enough to sand and buff without burning through. Again, I prefer to let this coat dry as long as possible, but I donít think you need two weeks as there shouldnít be much geography below. The longer this stuff dries, the harder it is to sand and polish, but it makes for a better reflection.
When I am ready to sand, I start out with 400 (which is pretty aggressive) and I work my way up to 600, 800, 1000, 1200, 1500, and then I use 2000 and 4000 pads. Sometimes you can get lucky and just start with 1200, but if there is a lot of sanding to do (relative to the 1200 grit) then you are likely to get frustrated with the lower removal and I tend to get more aggressive and that causes unevenness. It is crucial that the entire surface is treated the same. After 400, Iím wet sanding.
Finally, itís time to buff. I start 7Ē Makita buffer/polisher set to approximately 1500rpms with a sheepís wool buffing pad and 3Mís perfectitII compound (I prefer the 3M system as a whole over all others as a whole, there are some individual products that I like better). The longer dry times may require an extra cut compound. This is where the accuracy of your sanding pays off. Each step should remove the scratches from the previous step. If you fail to remove the 600 scratches with the 800, then they are there to stay until you go back and do it all over again - so be accurate. The compound should remove all sanding scratches, donít advance until they do. Different people have different preferences for buffing compounds and polishes. I usually use at least 2 grades of compound before using a polish. The second grade will be applied with a 50/50 synthetic/wool buffing pad which is still aggressive, but less so than the pure wool. At final polish, Iíll use the synthetic yellow pads, but make sure they arenít recycled automotive pads.
A few final words of caution - be careful with your buffing technique. As is the case with sanding, there is a tendency to hold the buffer at an angle to power out a troublesome area-this will affect your reflection. Use a different pad for each step, donít use regular compound on a pad you just used extra-cut on. I think most importantly to buffing, be careful of your heat. The hotter the pad gets, the better it cuts, is partially why the wool pad is more aggressive. However, the hotter you get, the better your chance of rippling your reflection. The heat will actually cause the film to have a broad ripple effect or a 7Ē swirl or is you are really aggressive and get it real hot at an angle with a dried out pad, you can make the compound part of your finish.
So, that is my way of achieving a perfect reflection. If it sounds labor intensive - it is. This is not a quick way to a perfect finish and while there are steps that can be taken to have a nearly as good finish for much less time and money, it will still be ďnearlyĒ as good and like I said before, gloss not done right, looks horrible. This is one of those situations where you get back what you put in.
Comment from contributor K:
When properly braced, a high quality MDF is the best base material for speaker cabinets. Its density is ideal for an acoustic environment. Unfortunately, MDF does have its limitations as mentioned earlier (brittleness and does not play well with water).
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