Admac- using full strength coats, I'd be worried about the paint drying completely. One benefit to the thinned paint is that it dries faster all the way through. I have done a single coat of black rustoleum pro and mineral spirits, and after a day it was rock hard and unscratchable (as are the results of others here). But where I spilled black paint here and there at full strength, to this day I can slice it with my nail. I think when it goes on too thick, the top portion can dry before the paint underneath has had a chance to evaporate the solvents, and it stays soft indefinately since the top layer, which might be hard, is sitting on uncured soft paint underneath it..