I had my Saiga 12 Cerakoted. I just removed the trigger group, piston, and any other internals + or course the stock and anything else that would easily come off. Panhandle powdercoating removed a couple other pieces that were a bit above my confidence level ....they wanted to make sure they got every square inch well coated! I would NOT remove any rivets.
It cost me about $150 to have the Saiga coated (receiver and barrel)....and I would definately go the cerakote route again.