Take a look at the TDS sheet of the paint you'll be using and it'll usually tell you what grit they recommend.
As far as the surface, it all depends on what type of guarantee you want. Scuff the surface and shoot paint over it and you're rolling the dice on a reaction. No way to tell what will happen.
Me personally, I'd use epoxy primer (prep surface per that TDS sheet first) and then paint over that for peace of mind. So far I haven't seen epoxy react with anything and it kind of seems to give you a "start over from here" layer. Then again I don't paint all that often so this is just general DYI advice.