Originally Posted by
tlracer
If they're magnesium then having them chromated is the best base surface finish to prevent oxidation. In fact these look like they may have been chromated already, going by the dull grey surface.
Magnesium alloys - especially old ones - can be very porous and 'gas' badly with things like powdercoat because of the heating involved.
With the seat repair, a good wheel guy might be able to weld/recut. Alternatively the damaged area might be better machined to an inverse cone, then have either equal cones on the back of the nuts (either machined solid with the nut or as a separate washer.
Just my two penn'orth (as we say this side of the pond...)