well when i did mine i used a wire brush and thinners to both clean and slightly score the rocker as the rough surface makes the primer adhere better below are the steps i took to paint mine hope it helps:
1. using thinners and a wire brush i scrubbed the rocker cover to remove any grease,oil or dirt that was on it. whilst doing this it scratched the surface slightly and i made sure this was even over the whole surface.
2. the primer i used was hammerite special metals primer as i found the primer i used before can't remember the name of it didn't adhere properly and flaked off after being subjected to normal engine running temperatures. i let it dry 12 hours just to be sure.
3. using a very fine sand paper i then lightly sanded the primer to give another slightly scratched surface for the next coat to stick to.
4. i decided i liked the look of honda civic b16a engines and decided to recreate this effect. using b&q stone effect paint i then coated the rocker cover in a fine layer of this stone effect paint. Basically a paint with fine particles in it. i let this dry for 12 hours with gentle heat from an air gun to be absolutly sure it was dry.
5. using a can of deep gloss red i sprayed a light coat over the rocker cover whilst drying it with the hot air gun. i repeated this coat 4 times to build it up to a deep red sand stone type texture rocker cover. let dry over night around 16 hours
6. using a gloss lacquer to coat all this and protect it i built this up in 4 coats to give it a bit of durability. i then let this dry for 8 hours then carefully refitted to the car and let the engine run for half hour to see if there was any discoloration which there was none.
I know these steps don't take as long as stated but i was just being careful because im sure you have all painted something decided to see if it is really dry and put a finger print in a perfect job. After doing this tooooooooo many times i just make sure and let it dry for a long time.....