If it is mildew, you can use something as simple as a dilute bleach /water solution, but I'm a fan of 3M Mildew cleaner. Another great option is a 'magic eraser' - google it. It's available tons of places.
What kind of surfaces are the stains on?
As for protection afterwards, 303 products...
Digging up an old thread because I spent the day at the marina working on my boat finish today. I had let the gel coat oxidize again after my last restoration - I got lazy and didn't polish / wax for a couple of years and paid for it.
I switched products from 3M this time and gave a product...
I used 3M Marine Rubbing Compound 09004 first to remove the oxidation, then followed it up with the Finesse-It II Glaze 09048, and then put a coat of the 3M scotchgard marine wax 09062.
All product was applied using a rotary polisher and a white wool pad. I spent a ton of time reading on a...
Rotary polisher with wool bonnet (white) and the 3M products. Gel coat is very different than clearcoat paint, and to eliminate oxidation you need a rotary. I have a 27' LOA SeaRay that I spent nearly a week restoring the finish on a few years ago.
Here's a before pic showing the level of...