Spraying Conversion Varnish with an HVLP Gun

      Old hands walk a beginner through the basics of mixing, thinning, and applying conversion varnish. A good introduction. February 14, 2006

After some quality control issues with my finisher I have decided to try doing the finishing myself. I am spraying SW acrylic conversion coating through a Devilbiss JGA 510 w/ a 1.1 mm fluid tip. I get a lot of orange peel unless I lay on a fairly heavy coat, and I get lots of bounce back as well. Can anyone give me some tips or perhaps a reading list so I can learn how to do it right?

Forum Responses
(Finishing Forum)
From contributor A:
Try adding a recommended retarder for that product. It sounds like it may be drying too fast for it to flow properly. 3-5 oz. per gallon should do it.

From contributor B:
A 1.1 tip sounds kind of small to me. Try a bigger tip and less pressure. Itís not as easy as it looks. Thatís why Iím here every night trying to learn new techniques and tricks for different types of wood. Read as much as you can here and pick up a good book with pictures. You will never stop learning, and you will have new appreciation for your finisher and what he has to do.

From the original questioner:
I certainly never thought it was easy, but when you send pieces out for finish only have to remake them because they have been dropped, dented and scratched, that is just wrong. I am not sure if I can put a bigger tip in my gun. Does anyone have experience with that gun? Can you suggest some easy to read books that are good and have lots of pictures?

From contributor A:
Do you have any retarder solvent hanging around? If youíre not sure what solvent is what, list what solvents you have and I may be able to help you. I think adding the retarder will help you.

From the original questioner:
I don't have any retarder but I am going to pick up a gallon of SW K27 lacquer thinner. It is the slowest I can get on a Saturday. My spec sheet recommends K30 but it is not available today and I have to spray this weekend to keep with my deadline. SW regular lacquer thinner is K20 and from what I understand, the lower the number, the hotter the product. If I want to reduce by 10%, is that by volume Ė for example, 1.6 oz reducer to 1 qt finish?

From contributor A:
No! 1 qt. = 32 liquid ozs. 10% by vol. = 3.2 ozs. Before you get the thinner, check with them to see if they have any MAK, xylene, or butyl acetate. The MAK would be the first choice, then the xylene, and last the BA. Any of these solvents will be slower than the thinner. The thinner may help some but I am afraid it will still be too quick. If thatís all you can get, then thin about 20%. 6.5 oz per qt will work and apply thinner coats and an extra one.

From the original questioner:
I was thinking in pints and typing in quarts. I am not sure about the MAK, but I can get xylene at the local Home Depot. Is there a concern about quality of the product? Will these products have any effect on the quality and durability of my final finish?

From contributor A:
No, the xylene will not hurt your finish. It is what most CVs were made with for many years until the HAPS compliancy regulations came into effect. It is not a HAP (hazardous air pollutant) compliant solvent. If you were a big user you would only be allowed to spray so many lbs. of any finish containing this solvent per year. It will help level your finish out I am sure. I would mix about 20% into the finish for max leveling. Just put an extra coat on it if you need to. The other thing not mentioned above that may be a factor is the air movement over the piece. Make sure you remove the pieces from any high air movement environment as this will skin the surface and stop it from flowing out properly. Air movement after it flashes off (about 20 min.) is fine.

From the original questioner:
My product is an acrylic conversion coating (S-W T77F62). Is there any difference between that and CV or will everything we have talked about work with this product as well? Also, naphtha is in the same family as xylene but is slower. Would that work too?

From contributor A:
If it is Hi Flash naptha and not VM & P you are correct. Both the Hi Flash and xylene should work fine. I would use less of the Hi Flash, maybe the 10% we mentioned above. I would also run a test panel first.

From the original questioner:
I got the xylene and am about to give it a try. Thanks again for all your help, and if you know of any good books dealing with this I would be interested. Just as long as they don't contain too many big words!

From contributor C:
A 1.1 nozzle set? Match the viscosity of the finish to the fluid size. A 1.1 is small. That means you may need a thinned down finish, perhaps 20 seconds using a Ford #4 cup. Or, one drop per second as the finish drips off the mixing stick. I would rather lay down a thinner finish than extend the open time (flash time) of the finish. The longer the finish needs to become dust free, the more dust and trash are going to land on that wet finish.

Pick up Bob Flexner's newly revised and revamped "Understanding Wood Finishing". It tells you how to optimize your gun (adjust the amount of atomizing air). Measure how thick your coating is that you are laying down. If you are putting on more than 5 wet mils then you are spraying way too heavy.

Did I read that your coating thins with lacquer thinner? If that's the case, what are you doing playing with xylene or even with naptha? Don't play alchemist or formulator. Just thin down your coating, dial in the right amount of atomizing air, hold the gun about 5 to 8 inches from your surface to be sprayed and lay down a wet coat that is more than 3 wet mils but less than 5 wet mils.

From contributor A:
I only want to know if it works ok.

From contributor D:
Hopefully you ran that test panel because by now you have discovered that the xylene definitely did not work in your coating. SW's T77F61, 62 and 63 Acrylic Conversion Coatings are not conversion vanishes, so they don't use the aliphatic hydrocarbon reducers like toluene, xylene, and hi-flash naphtha. Rather, they use MAK as a retarder and SW's R7K120 lacquer thinner as a reducer. You would be OK with the R7K27 retarder thinner (pricey stuff), but following contributor Cís advice would yield the best results in your current situation. Also, I wouldn't trust the purity of any solvent from a box store. By the way, the SW store that sold you the T77F62 has a Technical Data sheet for the product giving all of the above info, and if you use SW products with any frequency you should get their Chemical Coatings guide to have on hand for handy reference.

From the original questioner:
After a little more study and a bit of a drive I found a can of K127 Thinner. I was going to spray test panels using both. How much will the retarding thinner delay the flash time? I have also read that retarder is good to use if it is colder. The makeup air in my spray area is unheated and we are expecting a high in the 50s tomorrow. How will that affect the process? If I just thin with lacquer thinner will that change the flash time?

From contributor D:
The evaporation rate (the time in minutes that it takes for 90% of the product to evaporate) of the R7K27 retarder is 33 minutes; the E.R. for MAK (R7K30) is only 25 minutes. Use no more than 4 oz/gallon, or you're asking for trouble. This isn't enough to reduce the viscosity of the coating to the point where it can be sprayed through a 1.1 tip with good results. Also, since the T77 is an acid-catalyzed coating, you're going to have to keep the drying and curing temperatures above 65 degrees F.

It seems to me that you're in a bit of a corner here. If I was in your position I would call the client/contractor today, outline the situation, and explain that as soon as you get your hands on the proper reducer (Monday, hopefully) the job will get done. Otherwise, you run the risk of having the job rejected for poor quality, or having it fail a few months down the road.

From the original questioner:
By proper reducer are you referring to regular lacquer thinner K120? I have some of that.

From contributor D:
K120 is it. Catalyze the unreduced T77 as per SW's instructions, then reduce as necessary (20%, maybe?) to achieve good flowout. Make sure you're not running too much atomizing air (no more than 10 psi at the aircap), which would cause the coating to crawl from the air blast, and follow Daniel's advice re wet mil thickness. You should be good to go, but run that final test panel just to be sure.

From the original questioner:
All right time to get it done. I will let you know how it goes.

From Paul Snyder, forum technical advisor:
It looks like you're getting on track. Here's a quick list of the steps to successful spraying:

Make sure your compressor supplies enough air, in CFM for your spray gun.
Measure the viscosity of the finish you're planning to spray and make sure the spray gun has the correct needle/nozzle and air-cap installed.
Set the air pressure, fluid flow, and fan width on the spray gun to obtain good atomization at the flow rate you're comfortable with.
The amount of air your spray gun needs can change depending on whether it's a siphon feed gun or pressure fed as well as the air-cap that's installed. The correct pressure at the gun with the trigger pulled halfway is important to good atomization.
Here's a couple data sheets for different set-ups for the JGA-510;

57 air-cap
98 air-cap

To measure the viscosity, follow the procedures at this link. Related Web page: Viscosity At the end of the article, there's a link to information on the basics of spraying that may be helpful. Once you have the viscosity, select and install the fluid tip, needle, and air-cap that will atomize the fluid in that range. Here's a link to a generic chart. Related Web Page: Viscosity and Wetmils You'll have to contact your DeVilbiss rep to find out what their selection is. If you have problems setting the adjustments on the spray gun, the trouble shooting chart in the spray gun service bulletin can help you straighten it out.

Also, when it's not possible or convenient to install the needle/nozzle size that matches the viscosity of the coating you're spraying, the quick and easy solution is to thin the material until it works well with the needle/nozzle in the gun. Thinning reduces the solids content and makes the dry film thickness of each coat less than it could/should be. That means you'll have to spray extra coats to get the film thickness of the unthinned material. In a pinch, it's worth it to get the job done.

From the original questioner:
I am not sure how much versatility there is in the fluid nozzle size for my gun. I plan on contacting DeVilbiss for more information. On closer inspection I realized I don't have any K120 thinner. What will happen if I reduce using either Startex or Kleenstrip brand lacquer thinner? Does the quality of the product vary greatly from one manufacturer to the next?

I have the #98 air cap, but it looks like I can purchase the #57 cap and have a greater choice of fluid sizes with the 510 gun body. It also looks like the JGHV-531 gun uses the same body as the one I have. If so, I should be able to replace the baffle and air cap for an even greater selection. Does anyone have information on this?

From contributor E:
I would strongly recommend using the manufacturerís thinners and retarders and whatever else you need, at least at the beginning until youíre more comfortable with what youíre doing.

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