you need to go to a motorfactors/ chandlers and get some paint called polyester spraying filler, mix up as directions and spray on a couple of coats, if you dont have access to spraying equipment it can be brushed on you will just have a bit more sanding to do, once this is done and flattened down you need to prime it with a celulose primer / aerosols are available the flatten down again with wet and dry, then use your topcoat,