I scrubbed and scrubbed at the soap scum on my bathtub. A fingernail could get it up, but scrubbing with general purpose household cleaner got nowhere.
WD40 works pretty well. Spray it on, scrub with a diaper. I read on a boaters forum that there is some concern that, as a solvent, WD40 may emulsify and remove some layer of the gel coat.
Things to try next time:
- Citrus cleaner
- Borax (with soft bristle brush on the non-slip surface)
I may try The Original Gel-Gloss Kitchen & Bath Cleaner & Polish or this repackaged edition or I might just put on a marine wax.
Maybe this: http://www.thecleanteam.com/Fiberglass-Cleaner-16-oz-spray-top-_p_72.html