I want to spray finish, but I have heard that it is not a good thing to use thin varnishes before applying. Is that true?
Greg Williams: Thinning the varnish a bit won’t hurt it, and is often recommended, but varnishes dry very slowly, and are prone to sagging and runs, especially when sprayed.
Kevin Hancock: It is fine to thin varnish, but it will take more time and coats to get a nicely built finish. Many spray caps require the material to be thinned to make it through the spray tip. Experiment a little with different mixes to get your equipment to spray a nice coat without thinning it too much. Do not spray heavy coats or you are likely to get runs or sags.
Michael Dresdner: If we are talking about oil-based varnishes, it is perilously easy to get sags and runs when spraying these slow-drying finishes. To increase your odds, start with the fastest drying varnish you can find, and thin it substantially with acetone. Thinning will not harm the finish, so don’t be afraid to thin it 50% or more if that helps your application. That will allow you to apply very thin coats with a solvent that evaporates very quickly.
For vertical surfaces, this trick might help. Holding the gun a foot or more away from the wood, spray a very light mist or fog coat on the wood. Let it dry about 10 minutes, then go back and spray a very light wet coat. The tacky mist coat will help hold the light wet coat and make sags and runs a bit less likely.
Richard Jones: Yes and no. By thinning you can reduce the drying time, but you also get less solids onto the wood so coat thickness buildup is slower. Waterbased varnishes are a bit sensitive to thinning, but oil-based varnishes are less so.