I'm fairly sure that when I fitted overhaul kits to two servos that the interiors were good and I spray painted the outside surfaces only (to improve appearence).
From new (as you know) the servo body is cadmium plated and not painted the reason for this has to be the corrosive properties of brake fluid if a leak occurs and/or for a smooth surface finish.
Jim, regarding your rusty vacuum chamber, a good wire brushing and brush paint with a good quality rust killing paint ought to do the trick (this won't flake in the unlikely event of brake fluid contacting it and paint flakes could damage diaphragms in servo and reaction valve) - I think that it is important to have a smooth inner surface for servo operation.
If you don't like the idea of refurbishing your own servo (if it doesn't work you'll know about it because you have to press a lot harder on the pedal but your brakes don't fail!) then I suggest buying a reconditioned / new unit.
Why not 'phone the club expert on brakes as listed inside the back cover of our magazine to put your mind at rest regarding diy overhaul? Hope all this helps.
Regards, John.