What can I use to thin oil-based polyurethane for spraying?
Andy Rae: You can use mineral spirits, turpentine, or naphtha. The latter speeds up the initial flash time so there’s less opportunity for runs and sags to develop. I recommend starting out with 20 percent thinner and adjusting up or down from there until the finish atomizes, flows and levels out to your satisfaction.
Greg Williams: Use the thinner (reducer) recommended by the manufacturer for that coating. Typically, this will be something in the class of mineral spirits or naphtha, but could be xylol, toluol or a blend.
Michael Dresdner: All of the above solvents will work, but in my opinion, they dry too slowly for effective spraying. I’d suggest acetone. It dries much faster, making it easier to get a sag-free coat on vertical surfaces. Thin it heavily—50 percent or more—so you’re encouraged to apply very thin coats. Incidentally, the acetone may make the polyurethane appear cloudy in the cup, but it will dry clear.