Try the International paint site they may make some sort of barrier coat.If it was my boat I would get the hull grit blasted back to the gel coat and start again with a two pack poly paint.Your not likely to get blisters above the waterline more likely scrapes and scratches if the boat is the right money and your surveyor gives the hull as structurally sound go for it these are excellant boats well worth the time and effort to restore.With oil based paints you could try caustic soda as a paint remover by all accounts this does no harm to gel coat,it's the active ingredient in Dilunet antifoul remover.I read somewhere that people mix the caustic soda with wallpaper paste then brush on and leave for 24 hours and wash off with water.