I have a HVLP gun and want to spray rather than brush polyurethane. I know it can be sprayed because I’ve seen it sold in aerosol cans. What type of thinning ratio and air pressure should I use?
Michael Dresdner: Use a fairly thick, fast-drying polyurethane, and thin it 50/50 with acetone (instead of mineral spirits or naphtha). Yes, there really is a difference in drying speeds from one polyurethane to the next. For example, Varathane Professional (black label can) dries considerably faster than the same company’s finish for furniture, Varathane Polyurethane (wood tone label), which in turn dries faster than their floor finish.
Reducing with acetone will give you a fast flash off, and allow you to apply very thin coats, both of which are important for spraying. As for technique, on vertical surfaces, spray a very light fog of finish onto the surface by moving the gun back away from the piece, and setting it with lots of air and very little material. Let the fog or mist coat set up for about 10 minutes, then spray a thin wet coat. That will allow the mist coat to get tacky, and will help hold the thin wet coat so that it is less likely to sag or run. But no matter how you slice it, the critical issue is to spray sparingly.