RE: Rusty stain around screws.
Thanks for the chemistry tip. This reminded me of yellowing hull and how to make them white again.
Acetic acid in commonly found in vinegar. You can try citric acid as well which is commonly found in lemon juice (less stinky than vinegar).
Anything which is weakly acidic will works to remove singly positive charged salt (Na+, K+, etc...) and doubly positive charged salt deposits (Ca++, Mg++, Cu++, Fe++, etc...) commonly found dissolved in fresh and salt water. Some freshwater or saltwater windsurfing areas may contain higher or lower concentration of each of these mineral salts, hence producing different degree of hull's "yellowing".
The mechanism of "yellowing" or "rusty stain" removal is by chemical competition with H+ protons found in the acid. The higher concentration of the acid is , the higher number of H+ proton is found and the quicker the competition reaction goes.
It looks like the plastic surface of the hulls made by the Cobra factory is negatively charged to allow the binding of salt deposits by coulombic interactions. The cure would be to coat the hull surface with a "paint" containing a chemically inert agent. I guess a metallic ion such as Aluminum or Zinc could be used to render the plastic surface non-reactive chemically and corrosion-free. Unfortunately, I'm not educated in plastic engineering, so I can't elaborate further. The plastic bottle industry or the food plastic packaging industry should know the solution to this problem.
The same principles described as above do apply to cure monofilm sails which have turned white/yellow with time.