You just want to clean the valve cover? Or the head gasket surface itself? For the valve cover that has that clear coat peeling off, use aircraft stripper and it will melt off. Then you can polish up the valve cover, wax it, or clear coat it again. If you're pulling your engine apart and want to clean the head gasket surfaces, use gasket remover and a scraper. Gasket remover and aircraft stripper can both be found at most auto parts stores. For aircraft stirpper, I like Mar-Hyde Tal-Strip II. It works great but is nasty stuff so use gloves, eye protection, and do it in a really well ventilated area or use a respirator.