Once you've gotten the majority of the grime off a good final rinse with Acetone and a big stack of clean white lint free rags works great, follow up with a wash down with metal prep (the type that leaves a phosphate coating like Eastwood's "Ospho") rinse with water (I use dripping wet rags) until the stickyness of the Ospho is gone, then let it air dry. A nasty looking white film will be apparent when it dries, this is the phosphate that protects the metal from rusting short term. Once dry shoot paint directly over it, works great!