Originally Posted by Blizzard
Oxalic acid is the way forward.
Good rubber gloves and glasses / goggles. Anything that cleans that well is not kind to the skin.
Yes, please be very careful with Oxalic Acid especially if mixing your own as this has the potential for being much stronger than proprietory solutions. It is a very dangerous poison highly corrosive. You don't want it on skin, eyes, inhaled or, of course, ingested.
Also be aware that it is toxic to aquatic organisms too. It should break down fast in a marine environment though but use sparingly!
Also, wise not to use it with the boat on your galvanised trailer - it can attack the zinc leaving white oxide deposits!