The best way to do this is to use a strong and effective fiberglass boat cleaner. All of the products featured in this review can do the job very well, even when dealing with yellow stains. But you can go beyond just cleaning tough yellow stains. You can pick products that incorporate wax or polish in them so they leave an extra protective coat to keep the hull of your boat shielded from the elements. Again, many of the products in our review have this feature so you’re covered on that front as well.