I gathered from a friend that in time the wax needs to be stripped off before rewaxing to prevent early recurrence of the stain. He used cellulose thinner, and his boat certainly looked good, but for how long I don't know. Staining varies a lot from place to place. We stained badly every time we went through the Netherlands, but for the last few years in home waters I haven't had to treat the hull at all, other than simple cleaning and waxing.
Lots of good advice above, I would just add that with care you should be able to paint your boat without leaving brush marks. Maybe two thin coats of undercoat then two of top coat (sounds like you've got plenty). Applied with a roller then very lightly gone over immediately with just the tips of a soft brush should do the trick. Technique is called "tipping off", bound to be videos on the internet somewhere.
To get a decent finish with Toplac the preparation work is almost as onerous as polishing the gel coat. Refinishing large areas such as hulls is 80% preparation.
That's understood, thank you. Fortunately Toplac professes to need neither primer nor undercoat when painting sanded and degreased GRP. Also fortunate that GRP and paint are much the same colour. I may even get away with one coat but I doubt it.
In my experience, more often than not that’s a build up of old uv damaged wax and a wipe with alcohol removes it easily. The problem is often that people don’t think to clean before they try to wax, polish or paint.