My fuel filler cap ring was especially manky, so I gave it some TLC. The six lugs that attach the barrel casing to it about made me cry, but I eventually managed to pry it all apart (the trick: unlatch 3 on one side, tilt, pop; with the help of butter knives).
I used 120 grit sanding sponge to polish the ring, coated with 4 layers of silver Hammerite direct-to-rust for a brushed look, and a clear finish.
One more thing off the list