If you're wetsanding with 400 and then painting directly on top, make sure you get quality sandpaper or seal the surface before painting. Or get 600 grit paper. I've seen the sanding scratches come back through if the old paint is just sanded w/ cheap 400 and painted over.
I've always washed the parts, cleaned with a degreaser, then started the sanding. I finish-sand with 600 before painting.
Good luck. Pics when finished.
