sounds like everybody does this too. I have mounds of old bottles. Always useful as the guys said for mixing custom colors. (when I make custom colors I write the paint ratios on the bottles so I don't forget how to make them if they turn out well) All you need to clean out the enamel paint is enamel thinner or something stronger. Enamels are oil based so cheap lacquer thinner should work I believe, and if not you're only out a possible waste bottle anyway. They already said this but I agree that cheap powerful solvents are cheap. Kinda dangerous to breath in, but cheap. Let em soak for a few hours, drain them wherever you dispose of such liquids, and wipe em clean with somethin like non fibrous rags or paper towels. I use the blue automotive ones, they work the best and are durable. Like dog poo durable.