Powerwashing with straight water is the place I would start.
Thank you for your answer. However, this boat was left out in the elements while someone was going to sell it and it is awful. Mildew and mold lost its sheen. We've tried all kinds of cleaners for the fiberglass and nothing happens
Thanks for the answers, but I'm still hoping for more ideas.
I assume you have talked to local marine stores. There are all kinds of specialty boat cleaning products out there. Surely they have something that will do the trick.
Barbara, Sun (UV Rays) are the worst thing for fiberglass. UV's break down the epoxy and if left long enough, will destroy it. The mold and mildew I'm assuming is on seats etc? Bleach and water should help, but the stain is there to stay. As far as the hull, you might have to paint it or if its a valuble boat, re gelcoat it. This can be expensive. If its just a knock around fishing boat, I'd paint it and forget about it and enjoy it. They do make a paint specifically for fiberglass. Id look on line at boat supply sites and you'll find it. Most important thing is even if its not perfect, enjoy floating around in it
Most fiber glass is also covered with a "gel" coat...that provides the shine this "layer" of the finish is what protects the actual fiberglass below. As Harold mentioned above these can be recoated (gelcoat) to make them look like new...but this is where the $$$ is it worth it equation comes in.
Check with some local marinas and boat repair shops they should be able to give you some cost estimates...then you just need to figure out if it is worth in.
When we had our 25 foot sailboat my dad would joke that only way your can keep them afloat is by stuffing the holes with money.
You are at the point where at minimum, a re-gel is imminent. You may even have to bite the bullet and have to put on an additional layer of fiberglass in places just like you would have to do if you drove a corvette and have abused it and ran into a few posts over a period of time.
H-m-m... I just can't leave fiberglass product users just hanging if you truly bought or own a reasonably taken care of product made of fiberglass. If any of you fall into that category, there's cleaning hope for you if you are diligent.
Just remember what fiberglass is... It's porous, really!
It's cleanable, though. Don't start with store sold chemicals yet unless your situation has been cooking for far too long, like Barbara's!
Here's your solution: Soap and water! Yep... that can even be something like dish soap. Use it not just every once in a while but every single time you use your fiberglass thingy. If you haven't used it in a while... Wash it again anyway. Fiberglass looks great new! The thing about it is that even with no use, the sun, wind, weather will ALWAYS effect the finish...
Oh and remember to rinse, rinse and rinse.