Very interested in these posts. I have just acquired a boat which exhibited something of the opposite to the blisters you describe – quite large clusters of small ‘pitting’ – 2-3mm diameter each and various larger scratches and ‘craters’. This was only on the deck and not on the hull. I have reasonably successfully filled all the areas in question with gel coat filler, wet and dried and then painted. I dont know what has caused this damage – I kind of hope it was just sun damage and that it is not anything more serious. Picture attached (hopefully!) does anyone have any ideas ?