Hi Brian, the crackle paint came from Frost, I can't remember if it was their own brand or not. I cleaned the heads with an abrasive wheel first (one of those made from scouring pad type material) then degreased them, again with a Frost product. Then put the oven on to about 100oC, cover a baking tray with a couple of layers of tin foil, put rocker covers on and put in the oven for about 10 mins.
Take them out of the oven and out side to spray. As the rocker covers were nice and hot the paint crackles up straight away, so you can give it a good couple of coats. I then put them back in the oven again for another 10mins, took them out gave them another couple of coats of paint and baked them again! As they were hot the paint drys really quickly, a hour or so later they were on the car!
Hope this helps. Ade.