Polyurethane congealing problem

[jim gossage]

I am having problems with my polyurethane (Minwax fast drying satin) congealing during brushing onto a walnut picture frame (2x3 ft). The frame has a moderate contour and was sanded to 240 grit before finishing. This is a brand new can of poly. The first coat went on without problems. I cleaned my brush with Crown odorless mineral spirits (new can) after the first coat. The poly was dry to touch in 1-2 hours but I waited about 3.5 hours before brushing the second coat. To brush the second coat, I laid down a medium thickness layer of poly into all the contours of one side of the frame, and then went back over it with the brush to remove any excess and smooth things out. During this smooth out process, small lumps of congealed poly would appear and accumulate on the brush. I could sweep them to one end of the frame and clean off the brush on a dry towel, but it did leave the surface a little lumpy. I have used poly before on many projects and never encountered this issue before. The finishing was done in my airconditioned shop at a temp of about 77 degrees. I did not sand between the coats, but in past projects I often didn't sand until before the last coat. It cuts down on sanding time and the final product seems the same to me as if I sanded between all coats. Any suggestions as to where I am going wrong? Thanks much.

[Conrad Fiore]

Jim,
It's possible that you may have rushed the reapplication time and the first coat had not dried enough to resist redissolving by the thinners in the second coat. With poly, you should follow the directions for dry time and sand between coats for adhesion.

[Howard Acheson]

Two things that are important with that finish. One is that coats should be applied very "thinly". Second, 4-6 hours should be allowed between coats. Both of these points are covered on the label instructions.

Finally, this is a very fast drying product. You should apply in a single application and not leave a freshly applied coat and then come back to smooth it out.

[Bob Wingard]

Are you dipping your brush directly from the can ??? ... DON'T ... always pour off what you need into another container ... never return even the smallest amounts back into the original container.

[jim gossage]

Thanks guys. I did follow the directions on the can which for this product recommend 3-4 hours before the second coat, and I did work from a secondary container. I waited overnight to apply the third coat and did not have the same problem with that coat. The only direction that I didn't follow was sanding between the first and second coat. Could that have been the culprit? Re "coming back to smooth the coat out", this was done within 2 or 3 minutes of laying it down. Since this is a frame with coves and grooves, I find that even a lightly applied first coat tends to "pool" a little in the bottoms of the grooves after it is first brushed on, so I need to run the brush over it lightly to remove the excess that has collected a minute or two later. But this is done before I move to the next side of the frame, so it still seems wet. Thanks again for your thoughts.

[Conrad Fiore]

Jim,
I think you figured it out on your own by allowing the finish to dry longer before overcoating when you didn't have a problem when you applied your third coat after allowing an overnight dry time. Curious though that the directions on the Minwax website call for a 4-6 hour dry time before re-coating. http://www.minwax.com/wood-products/interior-clear-protective-finishes/minwax-fastdrying-polyurethane
Poly really does call for sanding between coats for inter-coat adhesion purposes. I'm sure you have read Scott Holmes on many posts where he says poly doesn't like to stick to anything, not even itself. Minwax recommends sanding between coats exactly for that reason.

[Prashun Patel]

Was your brush thoroughly cleaned? I've not experienced poly not adhering to itself, but my instinct is that if this were the case, you'd see beading or globbing - not particulates. Also (and again, I'm no pro) but if the surface were not sufficiently dry, I don't think you'd see dry particles coming up; I think you'd just see your surface taking much longer to dry.

Also, if there was some kind of incompatibility, I don't think you'd be able to draw the particles out with the brush; they'd keep being generated until the solvent evaporates and the coat dries.

Without seeing it, my humble guess is the brush had some residual poly in it that dries, and is getting laid down with the poly onto yr surface. There might not be that much in there, which might be why yr able to brush it off the surface??? Perhaps you just cleaned it more thoroughly between the 2nd and 3rd coats which is why you didn't experience it with the next coat?

When I first started finishing a few years ago, I used a decent amount of poly, and hardly sanded between coats. I never experienced adhesion probs. I also do a decent amount of turned pieces with wipe on poly, and don't sand between coats. I wonder if this is a problem that doesn't manifest that often...?

[Mike Null]

I inclined to think that the humidity was higher than you realized which would have slowed the entire process.

[jim gossage]

Curious though that the directions on the Minwax website call for a 4-6 hour dry time before re-coating.


Very interesting. I went back and checked my can and it definitely says 3-4 hr. Seems like minwax needs to get their story right!


Was your brush thoroughly cleaned? I've not experienced poly not adhering to itself, but my instinct is that if this were the case, you'd see beading or globbing - not particulates.

I didn't mean to indicate that there were particulates - the congealing looked more like small globs. I was able to sweep them off the surface with the brush and wipe them off the brush with a towel. I cleaned the brush with solvent after each use, but perhaps not good enough. And it was humid that day. Sounds like one way or another, the first coat had not fully dried and I pulled up some blobs. I'll update after I do my second frame in a week or two. Thanks for the insights.