Thinning of Polyurethane

Thoughts on thinning polyurethane for spraying. November 25, 2008

I have been told that polyurethane should not be reduced and sprayed. Is this true? If not, what is the best way to do this? I am presently using Minwax polyurethane. I am refinishing a kitchen table and had a problem with brush strokes on my last attempt.

Forum Responses
(Finishing Forum)
From contributor C:
You can thin it. Wipe-on varnishes are very much thinned and still work well. Spraying polyurethane is troublesome because of the slow drying and the sticky overspray that will coat everything in your shop. Spray it outside if you can. Slight thinning and a fine bristled brush with good tipping off technique could work well too.

From contributor A:
Many of the newer varnishes and polys have little solvent content due to VOC standards. The cans state "do not thin." For instance, McClosky Man O'War is an excellent exterior varnish. They reformulated it with less solvent. It is like honey. You should always use a sealer coat when doing varnish. The tech department explains the above and then tells you off the record to thin the varnish 20% with naphtha. Likewise they have reformulated the Minwax products. I know a couple of guys who have had application issues with the newer product. I would thin it up to 20% with naphtha. In order to spray it this is absolutely necessary. Don't use mineral spirits - it will slow down the dry time too much.

From contributor M:
In addition to thinning, I use a foam brush to flow the poly on thick, brushing (actually lightly drawing a full brush) from edge to edge. Have to do it early in the morning here in Colorado because of heat and no humidity.

From contributor B:
I've been using foam brushes - they work better than any real brushes I ever used, and I don't have to clean them. But about the thinning - doesn't that kill the gloss level? It sure is funny trying to brush out honey. It really is that thick sometimes.

From contributor M:
Hasn't bothered any of mine.

From contributor G:
Try the wipe-on poly. It's thin enough to spray out of the can.

From contributor K:
I do this. First 3 coats 50/50 gloss poly and paint thinner. Last coat 75/25 satin poly and paint thinner. I wipe it on with a soaked white lint-free cloth (old tee shirts work great). Roll it up in a tube shape about 3 inches wide. The poly flattens out so no brush streaks. If your project is going to get heavy use, just put more coats on. Drying between coats has never been a problem.