Picked up this ratty side cover up on Ebay for $10 because it had an emblem . Going to refinish it. The top of the side cover has bare plastic with splotchy remains of paint. What is the best method to knock the cobbled surface down and get it ready for primer?
Probably 400 grit sandpaper, but only after you wash it really well with soap and water. You don't want to embed any contaminants in the paint. Otherwise, you may have adhesion issues with your primer.
Make sure to remove any loose paint before you primer.
320-400 grit finish is usually used before primer to increase adhesion. Some will recommend 800 grit.
Read the data sheet on your paint system.
81-KZ440-D2. Louis Dudzik's GM HEI ignition module converted 2015--no issues, Motogadget m-unit blue.
Sandpaper for sure but not 400 grit unless you got hours of time for sanding. I'd go with 220 or even 150 or thereabouts. Don't sand too deep into the plastic otherwise it will take more layers of primer to fill the scratches. I like to remove as much paint as possible before priming. It takes longer but lower film build is a good thing.
In case you haven't started yet, try spraying Aircraft Remover on it and let it set. That stuff took off the factory paint on my 900F tank. If sanding, start with about 220 to 400 then go 1000 and 1500. If you don't you'll see all your sanding marks after you paint it.