Best I came up with to clean and brighten the machined faces of the engine fins is scotchbrite pad soaked in vinegar. Do one fin at a time and rub with gentle easy pressure so you don't wrap the Scotchbrite over onto the black powder coat and take you time. This is a couple hour kind of cleaning job but well worth it.

 

I bought a fine hobby file set, they are relatively small for getting into tight spots and the set comes with all sorts of angles on them. I did each fin individually and they look great. I used the files because I thought the use of any sandpaper was going to round out the edges and I wanted them to keep the sharpness. Once the metal is pitted, it's the only way to get enough material off to make them look new again.

Steve

 

I just use a bit of steel wool, cleans em up real nice. been about 3 years since I last did it, should get around to doing it again sometime soon......