Because the DP buckets are vinyl what I would do is avoid using seriously course wet and dry paper as you may end up putting some serious scratches in the surfaces, take some 180 grit sandpaper and work your way round the helmet, you don't have to go crazy rubbing seriously hard as what you want is a good surface for the primer to adhere to, then take a finer sandpaper and go round again to smooth it all out. Once your happy with the surface give the helmet a wipe with a damp cloth to remove any excess dust, you dont really want that sticking to your primer. What I tend to do after that is give the helmet a few hits of primer from about 2 feet away, go round but don't do it all in one coat, leaving a few patches is ok, let the helmet sit for an hour or so in between hits and repeat until the helmet is covered
