I'm going to be doing this myself soon pimping up a old strat copy. I work in a car paintshop and the way I'll be doing it is to lightly scuff the surface with 800 or 1000 grade wet and dry or sanding disc. Clean it over and when dry spray a coat of a plastic adhesion promoter over it (you may be able to get this at a good vehicle paint suppliers in an aerosol can we have it in this and for use in spray guns) It takes about 20 minutes to dry and then spray over with a colour base coat and finish with clear. Because the pickguard is flat and wont flex itshould be enough. Hope this helps.
