I like "Bulldog" adhesion promoter, it comes in either an aerosol can or
by the quart can for putting in a spray gun.
I prefer to use my gun to spray it.
That's for any bare plastic.
After that I always use epoxy primer if I want super good adhesion.
that really makes a difference for items like bumpers in reducing
road rash. Not all epoxies are flexable, I use one by SPI that is.