Just make sure the cover is super clean before you do anything to it. Then you have to break down the smooth finish so the paint will stick. I usually use 400 grit wet or dry paper which will leave light scratches on the surface. I follow that up with a Scotchbrite pad of 600 to 1000 grit, whatever I have around at the time.
Then clean the heck out of it again and spray with a quality ceramic based engine enamel like PlastiKote or Duplicolor. The paint will dry smooth over the scratches when applied right.
If you don't get the cover squeaky clean BEFORE you start sanding you'll stand a good chance of working the grime into the scratches as you sand and washing afterward might not remove it. The end result is bare spots and fisheyes in the paint.
