First three pictures are of my niece's car hub caps. You can see they lost their silver finish and it is flaking or peeling away.
Having done similar work on the hubcaps of a Nissan NX-1600 I started by:
1) Washing and brushing the hub caps clean.
2) Sanding the peeling material from the cap with 120-150 grit sand paper.
3) Using 220 grit paper with a Black & Decker Mouser sander.
4) Sanding the contour places with a sanding sponge at 150 grit and using 400 grit paper to reduce ridges on the surface. (To do a professional job one would probably need to sand blast the caps smooth.
5) Washed and wiped caps with sponge and cloth.
6) After drying, sprayed with plastic primer.
7) After drying according to primer manufacturer's directions, applied first coat of silver paint.
8) Waited till dried, applied second coat of paint.
9) After dried, inspected and did some touch up painting. Surface was metallic reflective so I treated with a clean sealer.
10) Sealer made surface duller silver and some what more coarse.
Here are some pictures taken of the hub caps after they were restored. The pictures taken at dusk were shot with the flash so there is quite a bit of reflection.