I've just painted my front bumper valance (the coloured bit) and used adhesion promoter first, then plastic primer. This gave very good results, so perhaps you could undercoat the trim before spraying with the stuff in the link (even though it says no pre-treatment required).
I would also invest plenty of time stripping any wax/forever black off the trim first. Perhaps a gentle steam clean? Or tar/solvent remover at the very least.