Polyester finishing system

Is it possible to use urethane/polyester in an HVLP spray system? April 4, 2001

I'm switching from lacquer and conversion varnish to a urethane/polyester finishing system on red oak. The problem I see is spraying the very thick (30 seconds in a Zahn #3 cup) polyester filler. Can this be sprayed using high-quality gravity feed (SATA) HVLP spray guns? Also, I live in a hot climate and a ten-minute pot life in an expensive spray gun is a concern. Anybody have experience with this? I know I can do it with a plural component Kremlin Airmix pump, but I don't have one.

Forum Responses
Polyester is impossible to spray with HVLP systems. It's just too heavy gravity fed. I wouldn't dare try it with a Kremlin, which is what I use for the majority of my spraying. Your best bet is a conventional cup gun, providing you have correct tip/aircap 40-45 psi. Don't be afraid to thin a bit (acetone). If you're able to spray it, you made a good choice if you're looking for high build. There is, however, a lot of sanding required. But what a beautiful look you can achieve!

There are other polyesters with longer pot lifes. Seagrave, Ilva and ICA have them. You can cut back on the cobalt to slow it down and many have a hot-weather cobalt, also. I have a guy that does full fill on ash with three coats of polyester sealer. There is no need for HVLP. The goal is just to dump it on. I use a gravity gun with a 4mm tip and yes it dumps it on!

The amount you spend on equipment to spray polyester should be dependent on the amount of poly you will spray. If you're going to spray a couple of quarts a day, use a 2 quart pressure pot with short 1/4" material hose. If you're going to spray a couple of gallons a day, you may want to consider a 1:1 plural diaphragm pump system that will work with any regular gun. With this system, you can spray hot (short pot life) and build quick without worry of pot life. Another thing to remember with poly sealer is that it doesn't work with all stains, and most of the time I use a barrier coat over the stain before the sealer.

Binks makes a plural component 2 quart pressure pot system. You will need to experiment with the catalyst ratio when using plural component systems, as the catalyst only gets mixed on the surface and you need more. I used this system and was quite pleased with the results. Plus, there is no pot life issue.

Polyester is one of the most amenable finishes to spray on. If I am finishing small items (car parts), I would just as well use my Sata gravity feed detail gun. It dumps the finish on very well and I do not have to clean up a larger gun. It also solves the problem of mixing up finish, so you can spray on your tack coat and leave the finish to set before your flow coats and before the finish turns to rock.

If I am spraying larger items, I go to my Crox/Accuspray turban. If you thin with acetone, it flows out very well. For large scale, mostly polyester primer, I use a Binks Gemini pump and a Mack 1, and as long as you keep new mix in the feel can, you do not have to worry about it hardening in 10 minutes. If you are going to spray all day long, then spend the money for the plural system.

From the original questioner:
I'm not going to spray very much polyester, since I hate red oak. Paste wood filler and I aren't on speaking terms. My SATA's are NR95 and 2000 models. I also have the MiniJets, which are spectacular detail guns. However, they are all HVLP and the polyester is just too thick for HVLP guns to handle, according to the above. I think I have a solution. I've got several conventional siphon-feed Binks Model 7 Taiwanese knockoffs that I'll use to hose this stuff on. If it hardens in the gun, I'm out $35.

I spray polyester just about every other day on auto wood dashboards, etc, and use a cheap $100 gravity feed HVLP with 2mm nozzle. Works great for me at 20-25 psi with the Seagrave products, and also with the much thicker wax polyesters. I can relate to the high temp problem--keep to 2% cobalt (if it's not already added) and use a less concentrated peroxide in the summer months.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
For poly spraying, I'd get a cheap knockoff of a gravity feed gun, HVLP or not. Woodworkers' Supply (I believe) used to sell a set of nozzles for the SATA lookalike that included 2.0, 1.7 and 1.4 mms. The nozzle you use depends on the thickness of the product, your desired pattern, and how it goes on, but don't be afraid to thin it as much as you need to get it to spray the way you want with the nozzle you prefer. (Of course, there will be more shrinkage with more solvent, so give the stuff time to cure well after an initial flattening.) To thin, I'd use MEK rather than acetone. Acetone can carry water, which contributes fisheyes.

The Seagrave product is pretty slow, so I can't imagine having problems with it (except for impatience when wanting to build more coats on vertical surfaces.) It is, of course, further slowed by reduction.