Just a suggestion that works on old fibreglass boats, pressure blast all the lichen and dirt off. Rough up the exterior gel coat and apply a two pack epoxy primer ( available from most boat supply stores like Whitworths (online). Wear a proper respirator. You can roll it on and tip it off with a brush. That will seal the exterior and prevent water getting to the glass. You can spray or roll on a topcoat. Or get a boat painter to come out and do it for you, but it can cost.
Have fun.
