As just pointed out, it is important that you find out whether the existing knobs are plated or truly brass. Manufactures have gotten pretty clever over the years in plating steel, plastic, and pot metal, none of which can be refinished. The easiest way to check what you have now is to use a magnet for plated steel and some lacquer/polish if it is plastic.
I have in the past found brass knobs at places such as Home Depot which were excellent replacements.
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