Acetone or denatured alcohol added to your epoxy is OK but no more than 5%. The most effective way to thin epoxies according to West Systems literature is with heat. Pre-heat the epoxy components to no more than 110-115 degrees before mixing (hot tap water in a pan is perfect), heat the area to be coated and have at it. The heat will cure the epoxy really fast so you should use some 1 hour epoxy at the least or preferably 3 hour so it will drain off or brush off to leave a thin coating to save weight. Pre-heating the area to be protected is the preferred way according to West Systems. Epoxy, unlike paint, does not get lighter as it cures because it has no solvents to evaporate. West Systems literature has everything you will ever need to know about epoxies and most of the info can be applied to any brand.