brake calipers restoring

Hi,
I'm in the process of building up my freshly repainted S. Next step is the brake calipers. It seems the alu. front calipers are fine, except for a small corroded section, on the edge with the "hole" where the piston sits. I guess the usual problem (between alu and steel), but what to do with it? Sanding the area and put blank paint on it?
The rear ones I would like to restore in gold-passivated finish. Do I need to take them apart (and refresh the O-rings), or keep them intact and put them to "bath" as a whole unit?

Thanks for the response!

There is a place in Washington State, US that does a fantastic job. I don't know if you want someone else to do this or if you even want to ship to WA. Maybe they will help you anyway with some answers.

http://www.goldlinebrakes.com/TechReport101.htm