I'd start with a good pressure wash, to remove dirt & grime, this will prevent from rubbing these things into the old paint. Then wipe down with a pre-cleaner (thinner) to remove oils, grease, silicones, super-sauces (Armor-all, etc.), sand or scotchbrite enough to scuff the surface paint, to give it "teeth" for new surface to "bite" into. Scuff first, paper & tape last just prior to painting (helps from sanding your taped edges & trapping sanding dust in your masking materials).