paint needs between 4 and 600 grit to really bite. Any higher than that is for finishing the clear coat. Primer will help fill in the scratches you put in, then you can paint or use an adhesion promoter if you really think you need to.
I would also mask it before sanding, then retape it if you need to. I used to be cheap with tape, don't. Your tape job can make or break your paint job ;p
Also, as far as how many layers of paint depends on what you are using, some stuff goes on thick, some doesn't. Just make sure it all looks even. You don't want to have to go back and re do it.