I have a question regarding the compatibility of various finishes on woodworking projects. Recently, I made some small toys with a colored enamel finish, and when I tried to spray on a clear coat, the base coat would wrinkle up. I also see some muddling when using stains and clear coats at times. Could you shed a little light on what base coats and top coats are compatible with each other, and which to stay away from? – Richard Gaudreau
Tim Inman: The answer to this is a college course. The short version is this: The problem is “solvent compatibility” (or incompatibility) between the finishes. It is the solvents, not the resins, which are the attackers causing your troubles. If the solvents in the next coat are strong enough to attack the resins in the lower coat, then interesting things can begin to happen. Things that smell like old-fashioned fingernail polish will attack and destroy about anything it lands on. Things that will burn are usually strong attackers. Things that don’t have much smell or won’t burn are usually pretty docile and won’t attack what they are applied to. But, rather than standing around with a box of matches and vaporizing yourself into a drunken stupor with paint fumes, here’s a better suggestion. Make a sample board for whatever finish system you would like to use on your project. Apply the various coatings in the order you would like to use them. Make the sample board a “step panel” where you coat the whole board with each new coat, except for a one-inch strip along the edge each time. When you’re done making your step panel sample board, you will have a story line of one-inch strips showing each new coat, what the effect is contributed to the whole, and the entire finish as it should be expected to show on your project. Yes, this takes a little extra time. No, it is not wasted time. Fine finishes are not accidental, nor are they serendipitous, happy accidents. Fine finishes are planned out and executed just like the rest of the project. Consider the sample board as the blueprints or working drawings for the finishing phase of your project – only tested in real time with the real things.
General rule guideline: Never put a stronger solvent finish over a weaker one. Varnish over lacquer, OK. Lacquer over varnish, NOT OK. There are exceptions.