first of all give it a skim of fibreglass, this stuff is the best,

you should be able to get it at motorworld, or local motorspares, then after almost sanding it off, gove it a skim of filler, isopon p45 easy sand is available from car spares again, and is ok stuff, then it's a case of high build primer and keep wet flatting it back with some 800grit, if it was bodyshop, it'd just get a coat of stuff called "dolphin glaze", it's like a liquid flexible super fine filler that finishes smooth enough to paint over, top stuff, but doubt you'll be able to get it, unless thru a bodyshop...
