RE: cleaned an engine in prestone and it blackened ... what to do ?
I have used this and an various brushes starting with brass and then a stiff nylon detail brush. Stainless brushes and this cleaner work well together but watch your edges and it might leave scratchs.
It's water rinse so it flushes out really well and leaves no residue if your careful. Works great with a scotchbrite pad. The compound really helps remove oxidation on brass without touching so it must be a little acidic.
Barkeeper's Friend is also a good product. Look for it in the sink cleaners at the grocery store, same with the copper cleaner. It's like Comet sink powder but different in the respect it etches aluminum nicely to a factory shade.
If you want to do it right it takes elbow grease and taking the whole motor apart. I don't know of any other way without masking and grit blasting.
One note about grit blasting, I personally would oil soak the cases afterwards to lift all the powder out of the aluminum pores with some good penetrating oil. I have done this and its amazing how much aluminum is left in the pools of oil.