Prep is prep.
If your going for best practice then stip off everything down to fresh metal.
Apply 2 -3 coats epoxy.
Do any filler/high build work needed to get your surface as perfect as your needs require.
A coat of sealer just before painting does two things.
Seals the layers below from moisture.
Provides a chemical bond for the paint to lock onto BESIDES the physical bond to the small scratches left from the scuff pad.
If your really serious check out "the perfect paint job" thread over at link to SPI