Hi Daz.
On my Portgarth GRP hull with wooden skeg I completely coated the skeg with a couple of coats of finishing resin.
I used the Halfords red primer for the whole hull and later gave the lower half a halfords satin lacquer knowing I could later apply the upper hull colour on top and ultimately I then lacquered over the lot.
I marked my water line with fine indelible pen on the primer before the first lacquer so that it largely remained through following fine sanding before adding the other colour.
You should notice that the lacquer changes the shade of red primer considerably and you either like it or you don't so just be aware
Still wet here below and changes quite a lot as it dries off.