Gelcoat is to a degree porous and the stains become embedded in it the only answer I found was to apply a wax coating each year which kept the hull reasonably clean and looking OK. I periodically and very lightly used a very fine abrasive used to polish cars prior to applying the protective coating which will deteriorate. I used oxalic acid locally to remove any staining prior to both the above treatments. It worked for me.Thank you for that, makes sense.
I've been told that a layer of oxidation builds up on the gelcoat and this needs to be stripped back by a "mopping' process to reveal fresh getcoat and this can only be done so many times. I'm hoing this is not the case with my boat.
As said above, the oxalic acid treatment worked really well on my hull, problem was the staining came back almost immediately after its re launch.
I am hope I was incorrect in re-polishing the hull, when I should have sealled it with a wax product.
I shall wait until the spring and redo the OA treatment followed by plenty of wax.
Boat is a 7m Rib that is in excellent condition other than its "dirty" looking bottom, lives in a rack during the summer, barn in the winter.