I started sanding my battery tray/box with a die grinder to knock down the large amounts of surface rust. I then detail sanded this area by hand. I primed the sanded areas with self-etching primer and then painted the area with dupli-color spray can paint. The main thing you want to keep in mind is what you are trying to accomplish. If you are trying to stop or halt further decay then this method will work. If you are looking for a show quality finish then I recommend you pull the engine and take your car to a good bodyman.
