Most of your silicone will simply pull off by hand in strips. Use a putty knife (or an official gasket scraper) to remove any that has found some teeth, but be gentle -- you're dealing with aluminum. Carburetor cleaner works great as a sprayable solvent, with narrow pattern if you've got a straw (silicone curls up like Wicked Witch of the West). Try not to spray on platic or rubber. No compression in a valve cover. Is full of oil, so you do want a tight seal. When you paint the covers, why not tape their mating surfaces? Bill Robertson #5939