Well you would normaly use wet n dry to key the surface so the paint can adhear to it but as it is soft plastic and scratches easily i just used the scotch bright pads when cleaning to take the glossy finish off the plastic and key the surface a the same time
They looked grey by the time i was ready for primer. You can probably get panel wipes from halfords or just get a bottle of panel wipe and some lint free cloth to wipe it down with. The panel wipe is the last step as it takes any grease out of the surface as even the grease from your fingers may let the paint peel. Once wiped with panel wipe leave for a few minutes for the solvent to come out of the plastic, it is pretty much all solvent so shouldnt take long.
If you want anymore info just ask.
Also a good tip is to leave the cans in some warm water for a bit before doing it and spray in a warm, wind and dust free place.
Dan