George is right. Sometimes, the best way is to simply use a very high grit sandpaper, which isn't all that different than using softscrub or a fiberglass
cleaner designed to cut the chalk.
I sanded all of ours with 220 grit when rebedding. I use the sandpaper on any of the stuff that wouldn't come off using other methods (such as Collinite's Fiberglass Boat