have repainted a few rims over the years with gutter rash ,i usually just sand them back with an orbital sander starting with an 80 grit and finishing with a 280 grit ,quick coat of 2 pac hifill ,let that dry ,rub back and paint the colour of your choice ,most gutter rash is not usually that deep .....if it is then it needs to go to a wheel repairer as just adding filler to a rim is a waste of time