Clear Finish Over Faux Linen Painted MDF Wall Paneling

      Here's an extended thread on an unusual and challenging architectural finish problem: getting a high-gloss clear coat over painted MDF in a fast-paced custom homebuilding context. June 8, 2008

Question
I am a decorative artist and mural painter and presently we are finishing a wonderful library in a private home. The finish is executed over a lacquer primed MDF raised panel room. This includes built-ins and wall paneling and crown mouldings, (basically top to bottom). The prep work in a fast paced construction site was such that all of the base coats on all elements are unknown. To avoid any material problems I have prepared the room and sprayed a high grade acrylic semi gloss warm grey paint.

The first and second "strie" or dragged finish is in alkyd glaze pigmented into a deeper warm grey. The first pass (horizontals) is thinned slightly with mineral spirits and penetrol for open time. The second vertical dragged pass is a lighter than base coat glazed grey alkyd producing the final "linen" effect.

I want to achieve a mirror like finish over this and we have turned the entire room into a cross draft spray booth that is filtered and well warmed. My chemcraft supplier has given me two options Acryllak 95 high gloss lacquer but warns that all oil based paint must be completely dry. The second is Nitro "old school" Lacquer.

My concerns for both are 1: the water born Lacquer adhering properly and being sandable and buffable. I have had this stuff peel off in sheets off of alkyd before. Itís possible that it was not completely dry in that scenario.

2: The reaction of Nitro Lacquer over such thick alkyd coats of glaze. Also, the ability to build the Nitro thick enough to cover any ridges left by the faux finish resulted in too many coats and inevitably cracking. Any help would be very much appreciated!

Forum Responses
(Finishing Forum)
From contributor C:
You can answer your own questions by first making up a sample and testing it out to see what the results will be.

That said 1: Nitro will yellow extensively over time and ruin the original appearance - if yellowing is a concern then I would tell you not to go that route.

2: Your choice of alkyd glaze medium could have severe repercussions, many alkyds are reactive especially to coatings solvents, which could cause lifting, crinkling, and other problems. They dry by themselves ok, but form a barrier to adhesion with many clear-coats. Many of them also yellow badly with time, though Iím not saying yours will - there are literally hundreds of different kinds of alkyds.

3: From where youíre at now the best thing to do is make a sample with every step you use and test it for adhesion and get a 300a UV lamp to check for yellowing 100 hours under this lamp will show 1 year in direct sunlight. This will tell you how the finish will look in a year. 1000 hours is equal to 25 years exposure, and so on.

For you - each step from this point on will need samples to test every step to know for sure the outcome - unless you want to remove the glaze medium and start over - not an easy task and I donít know your deadline - hope it's not short.

I would not go water-base over alkyd normally - too much chance of adhesion problems. You would be much better off with a high gloss acrylic solvent type, but you still need to test for adhesion problems to do that correctly you need to let the clear-coats dry for 30 days at least - do you have that type of time? If so - then I would recommend Delta labs 525 series of gloss acrylics but still test for adhesion - as I said if itís a reactive alkyd it may cause problems.



From the original questioner:
I have to thank you for your comments I appreciate your insight. As far as time goes we do not have the luxury of that sort of cure time.

I have a few more questions, Is there an oil based high gloss finish that can be polished to that sort of lacquer mirror like sheen? I generally spray Benjamin Moore low luster 1hour on oil based finished furniture in our booth here in the studio and love the product as it has a nice mellow feel. The yellowing properties will be fine in this application due to the warmth of lime stone and sandblasted oak flooring (herringbone pattern).

The main concern and initial reason to use lacquer was that high gloss mirror like finish. I have never had the need to polish the gloss varnishes before and have only done to date, final off the gun finished final coats with those products.

In an ideal scenario I would be looking for a fairly quick drying high gloss oil based
varnish that could be prepped for a final spray of that ilk. Perhaps there is an oil-based sealer that could be done first and sanded to achieve that final surface for a very high gloss finish.



From contributor R:
What products have you used in the past to achieve the mirror like finish your looking for? Time doesnít seem to be on your side and to mix a solvent based nitro finish over an oil based substrate is looking for problems for sure.

It seems as if your faced with using a product that is compatible throughout each step - that product is oil based. Oil based coatings donít dry very fast.



From contributor C:
Then you would need what is called a rubbing varnish - a short oil alkd like Behlenís four hour rock hard varnish, but Iím not sure who carries Behlen's anymore or if Mowhawk has it in their catalog.


From the original questioner:
The only times I have achieved this mirror like finish is with Nitro Lacquer and polishing it out. The oil based 1 hour Benjamin Moore finish does have a very high gloss but is elastic like all oil-based varnish. It will eventually rub to high level but not without tremendous effort and patience. The acrylic product from Chemcraft does adhere to oil-based glaze but only when the alkyd is completely cured. I thought I would try a sealer for the oil based glaze after 24 hours then sand and apply the acryllac Chemcraft product (sample only). I have been in this situation before but never to this scale!

The real problem started with my desire to have that soft linen finish and I do wish I could have used water-based glazes but the open time is not enough.

The workability for my specialty paint finish needs to be quite long and would be impossible to achieve with water-based glazes because of the size of some of the central raised panels. Anyone know of a sealer that will be higher slack and clear as a bell that is oil based?



From contributor C:
You may want to think about using Zinnser sealcoat as a adhesion promoter at this point - trouble with applying a varnish over an alkyd thatís dried hard without sanding the alkyd first is lack of adhesion. It sounds like sanding your glaze at this point would not be correct? If not then apply the shellac and then you can sand it lightly before you apply the rock hard.


From contributor R:
I have used Rock Hard varnish and it is a great product. Also you may do some ests with Target Coatings Oxford Super-Clear Polyurethane-9000. This is a great product, builds fast and buffs to a great gloss using Menzerna polishing compounds. You will have to depend on a mechanical bond to your substrate which will mean an aggressive sanding of your faux painting.


From contributor K:
We do glazing with oil glazes followed by waterborne finishes frequently. We have had success with spraying a light, wet coat of Zinszer Sealcoat (dewaxed shellac sealer) after 12-24 hrs dry time. Some glaze mediums dry slower than others. When spraying large areas we add a little lacquer retarder to the shellac. When dry you can spray your waterborne over the Sealcoat.

Waterbornes with ammonia in them burn into the shellac; other waterbornes may require a 320 scuff sand of the Sealcoat. We have used gloss to build up thickness and then topcoat with desired sheen. We have found that Polyvar by SDA works well for complex jobs that you seem to be tackling. If you are new to waterbornes, this would not be the job to experiment on.



From contributor C:
Ammonia does more than burn in to the shellac - it slowly deteriorates it. Ammonia in the early days was the stripper for shellac varnishes. Better to use an oil base over alkyd donít you agree??


From contributor R:
Target also makes a WB shellac and that is where my mind is going but not being the one who did the finish it's hard to know the best answer. I also do artwork however and have tested the Target 9000 over painted surfaces and was very impressed with the results. It is a cross linking formula but you don't add a catalyst. The clarity is excellent and it buffs out easily.


From contributor C:
I've stayed away from the water base shellacs because of their inferior properties as compared with solvent shellacs. Maybe I'll buy some and run some.


From contributor R:
I haven't used targets WB shellac either so that is why I didn't mention it in my original post. I have heard several people here have used it and like it. All of the products I have used from target however have been very good and I am not a WB guy either so it takes a pretty good product to impress me.


From the original questioner:
I am amazed and overwhelmed by the knowledge base and generosity on this forum. Thank you all! I am still in my studio spraying several samples of all of the suggestions listed. I will try to get some of that rock hard varnish for this project as well.

I will not be sanding the final glazed faux finish. So in order to protect all the work thus far I will have to get a get a good couple of solid and well covered coats before even a light scuff coat will begin. I am willing to lay a lot of material on over the next few weeks. Water sanding to approximately 1500 to 2000 grit before the rubbing compounds come out. I am in awe at my luck to have stumbled into this breadth of knowledge!



From contributor R:
I like 3M micro finishing film for my wet sanding. Available in micron sizes 80 micron all the way down to 9 micron. The 9 micron is the finest and the scratches will buff out very easy.


From the original questioner:
Hello everyone. I have ended up using a very expensive finish instead of the suggested. I used a Sherwin Williams Automotive product called ULTRA 7000 - their top line clear coat. It did not react and the build was incredible, as well as the hardness. We sanded the first coat and it was so thick and consistent that we ended up with on coat throughout the majority of the room. The buffing is easy as you can crank the buffer up all the way with a wool bonnet and you really would have to try incredibly hard to burn through any one area or spot. Problem is the room is almost too glass-like for my taste but the client is very excited about it.



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