When I had the top of my fiberglass pool re gel-coated this year, the guys cleaned the waterline with a very fine sand paper. It took the dirt right off - I know it may have taken a slight layer of gelcoat off, but the gelcoat is very thick. They left me some to use - it works really fast, and without any pressure it doesn't seem to hurt. I don't plan on doing this often, but what a time saver, and elbow saver. It may be something you want to try.