impossible to not remove silver by mechanical polishing methods, chemical cleaning depending on what you use will also remove a molecular layer of metal, although chemicals will clean they will not polish, I have pretty much done every method you can think of in my career, best is to just accept them as they are and don't touch them, if you must, use a FINE polish, eg car (not compound), jewellers white metal polish, autosol, ultra fine grade Alumina Oxide and water, ALL of these are abrasive and WILL remove metal although microscopic in amount unless you go bat shit crazy ass monkey mental with a buff wheel. Not really a practice I would bother with, or suggest. If you are talking scratched bars as in deep marks, just leave them be, sell them and buy minted bars or high quality new cast that have not been subject to years of handling, I only ever clean silver now before my refining process.