I use "Wet and Forget" on my shingle roof, soffits, garage door, and concrete drive. Each time it rains it cleans. I've used Simple Green on aluminum camper to clean mildew and also on the rubber roof. In early '90's, the Gov bought Simple Green by the 55 gal drum. Don't use AJAX bleach alternative on fiberglass. It will dry it out. It cleans good and maybe ok if you rinse very well then paint. But I would use Simple Green.

I don't know why Cobra uses the gelcoat they do. Maybe more flexible than what boat manufactures use. But I've seen boats out in the sun twice as long as a cobra trailers and looked twice as good. Maybe glider pilots don't wax their trailers. To me, Cobra trailers look more porous than boat gelcoat and oxidize way too quick for what they cost. And I have always wondered why an Aluminum cobra trailer top is a different white than the bottom. I like Cobra trailers, but maybe because there is not much competition. But they cost way to much.

I just ran across this that looks promising: http://www.biokleen.com/hull-cleaner

I noticed you said glasspaper instead of sand paper so you are probably not in US where some of these products are available. Since you are going to paint and have used a wire brush, something harsh like the AjAX bleach alternative or something like toilet bowl cleaner or muriatic acid may work but I would rinse very well. People use toilet bowl cleaner, or hull cleaner, or muriatic acid on boats to remove stains.

Good luck.