Use full strength denatured alcohol to remove oxidation. Have plenty of rags because they will soil fast with oxidized paint. I have done several boats by just washing them down with denatured alcohol, removes all of the oxidation and brings paint back to original shine/mirror finish. Good wax after will slow new oxidation.
The boats that I have done have all been aluminum hulls that were painted and the results were astounding, but I have not tried it on fiberglass. I would suggest you actually do a small test spot first. You may have to wash it down more than once to get it all off depending on how much it there. Remember that the waxing will also take some of it off as well so get it as clean as you can with the alcohol then wax. That paint will look like new!