A sheet of fine emery paper should do the trick, and leave you with enough remaing to give the tow ball a clean up for the whole season. I think the last sheet I bought was 17p
michael, on my first towball i used paint stripper (very messy) but on our new one i just used sandpaper then polished it off with wet and dry paper (wet) .i tend to keep it clean with t cut then wash with brake cleaner , works for me and no groaning when hitched up jim