at my work they use this 3M flexi pads, there super fin, The idea is to get it really smooth.
Id used the above, but your best bet is to use something 600-1500 wet and dry. use it wet, and wipe it clean with warm water. if you use wet and dry, make sure its really dry... heat panel up with something (not boiling hot) Theres nothing like painting on a wet surface trust me!
Also make sure all the edges etc are totaly dry and free from dirt. If your not in any rush get some degreaser.