Acid coil cleaner is by far the best product to use. It has a blend of acids to clean aluminum coils in air conditioners complete with surfactants. This is for unpainted aluminum. Don't use detergents that contain alkaline products as it will turn the aluminim black and if not washed properly eat a hole.

I purchase my coil cleaner at SWH supply in Louisville. Make sure you try it on the bottom first. Let it sit for a few minutes and wash it off with a hose. You can use a scrub brush if you like but I never do. This works great on all aluminum parts on a boat. It also removes scum off the bottom