I ran into some fenders like those a while back. I took them off the bike, soaked them in a oxalic acid bath (using a plastic wallpaper dunking tub found at a non-bigbox hardware store) to get off most of the stuff. Some fine steel wool with WD40 next to get any rough spots, and then I wiped some 3-in-1 oil to the surface. There was some minor pitting but one can't fret about that. Taking them off the bike allows you to clean the underside as well.

The fender stays are that grungy looking sheet metal stuff. Cleaning those didn't really make them look much better, so I painted them with a white rustoleum type of spray paint to match the trim and decal color.

The fenders looked really nice when they were finished.