First I scrape off the large growth with a scraper being careful not to dig into the prop. Then, I use emery cloth, which is basically sandpaper but instead of the sand being on paper it is on cloth. I start with 150 grit and then move up to 220 grit. I have a bronze prop and it brings back the shine. I then spray multiple layers of clear antifouling paint on it.
My boat comes out every winter and although its only in the water for a six month season, I still get some barnicle growth every year.