Another steel wool vote. At hardare stores you can it different levels of coarseness? 2x,3x, 4x etc. bigger the number the finer it is. I use 4x on windshields, cleans bad bugs, water spots and doesn't scratch. If the valve covers "aren't" pitted they should clean up. If it cleans up you can even put a coat of wax on em to repel moisture/water better.