Phosphoric acid which Ospho is a diluted version. Soak in decent temps several times, scuff with scotch brite, and rince good with plenty of water. Neutralize!!!!!the acid!!!!! Some will say add baking soda. Remember most epoxy primers can react to OSPHO if not neutralized and it can react months LATER!!! I would then apply 3 good coat of a good epoxy primer, level with GOOD filler, I would use a fiberglass reinforced filler, and THEN a good general filler , block, and do a heavy urethane sanding primer, or a polyester primer, block seal and topcoat.. If any imperfection you know it will show in the topcoat!! Or replace that roof!!!!
Vinyl roofs were the worst auto invention...ever!!
