The Mylar (chrome) film isn't easy to remove, it's bonded to the grill with a very strong adhesive.
The best way I've found to make paint adhere is to scuff the surface with a Scotch Brite pad (or fine sandpaper) to dull the surface, then clean with mineral spirits, not mineral oil.
With paint, prime, then topcoat. I've never used plasti dip on "chromed" plastic before, so I don't know if priming is necessary with it or not. With spray on bedliner a primer coat gives better adhesion, so if it was me I would prime first.