Silver contains metal filler and should be used in badly rusted, pitted areas. It will fill in holes better than black or clear. Black is generally used on frames, underside of fenders, etc. because that's the usual color there. Clear renders the smoothest finish and is often used as a pre-primer on exterior surfaces; Gray is great on concrete floors, bilges, storage areas, as well as steel.Note : Semi-Gloss sheen may vary due to film thickness and environmental conditions during the curing process.