Pinholes in Pigmented Poly
Troubleshooting a pinhole problem with an opaque poly coating — could it be solvent pop?December 6, 2011
Question
I’m spraying a painted finish (General Finishes pigmented poly) and I keep getting tiny pin prick marks when it is dry. There seems to be more pin prick marks when wet, and fewer when dry. Nonetheless it’s driving me crazy. I have four coats, sand between each, blow off with air, and still they are there. What is my problem? Do I need to buff them out of the final coat?
Forum Responses
(Finishing Forum)
From contributor J:
I don’t typically use poly but had this happen with precats and conversion varnish. If you are reducing the poly with a fast solvent you may be putting too heavy of a coat on and trapping solvent underneath (skins on top and traps).This causes solvent pop and it is very tiny - like you said pin pricks.
I hate to say it but you need to sand a lot of this out because all finishes generally will dry and take shape to what it’s laying out on. Also, try a slower thinner like spirits or 150 flash, or no reducer and lighter coats. You will just have to spray lighter coats. At this point I would be afraid of those pin pricks turning into air bubblss by trying to lay material on too thick to fill them. Also, does the sheen just make them harder to see?
From the original questioner:
I think that there are fewer once it dries. I did sand it down starting with 220 all the way up to 1000. Then I sprayed a thinner coat and it seemed to be better, although it had a slight orange peel affect with the lighter coat. I’m guessing that I have to get a little thicker than the light coat but not as heavy as the first two coats. I’m lucky this is a test door sample for a client, but I need it to show well and I have a lot riding on it. Would putting a retarder in help? I’m also using a WB finish from General Finishes. Would you recommend another finish? Until now I have been doing mostly stained cabinets, but times are changing around here.
From contributor J:
I have been spraying WB finishes for over a decade and the reformulated GF poly is among the best I have seen in pigmented coatings. First, I would check with your supplier and make sure you indeed got the newest formulation and not the original poly as that product was fairly marginal.
I agree with the other poster that too thick of a coating was my first thought on the problem but with WB's you might also want to look at things like humidity, air movement and temperature as all of them have been a factor in my area with temps ranging from 60-100 and the humidity bouncing from low to high. All of those things will impact the way the finish dries.
From contributor B:
I've just switched over from GF's acrylic to the poly and have found it to be super easy to work with. It seems it lays out even better than the acrylic. The only times I've seen what you're talking about is when I've laid it on too heavy. What type of substrate are you shooting on to? Are you using the undercoat before applying the Poly? What is your prep? The fact that you would sand it down to 1000 grit raises a huge red flag with me. You could easily have adhesion problems sanding it that smooth. To start you only need to sand your unfinished wood to 150grit. Then two coats of white undercoat followed by two top coats of the poly (sanded with 400grit and synthetic steel wool between each coat). I've used this recipe for years with nothing but good results.
