The Hobie Forum might have tips ( Hobie Cat Forums • Index page )Just got an old Hobie Cat as payment for a debt owed to me. The boat looks like it's in pretty remarkable condition with the exception of very dark discoloration below the waterline. I have a feeling this boat sat in the water for upwards of 10 years without cleaning. Any suggestions? Short of repainting?
There's another bunch of cat sailors at BeachCats ( TheBeachcats.com - Catamaran Sailing ) and ( beachcats : Beachcats, sailing beach catamarans. )
Much will depend on what the stain is - algae, motor oil and gasoline scum, mineral deposits, mildew. Each takes it own cleaner - there's nothing universal.
From my experience CLR works on some mineral and organic stains, but keep it away from any metal trim.
Scrubbing Bubbles works on other organics. Spray it on a dry hull - water lessens its strength - let it work about 15 min. before rinsing.
Oxalic Acid (wood bleach) will remove rust. A mild form comes in the cleanser Bar Keeper's Friend and you get the mechanical action of a mild abrasive too.
You can run through different levels of mechanical abrasives from Bon Ami hard soap granules up to Ajax with powdered silica (sand).
Beyond that you are into rubbing out or even wet-sanding back the gelcoat layer to try and find unstained material. Gelcoat is simply fiberglass resin without the glass fibers and with colorant for a cosmetic top coat. It's laid on 10x thicker than paint so you get a little room to sand down some. If you hit the fiberglass underneith, that's all she wrote - epoxy primer and paint is the last resort.