I do not know that this will be the "ultimate" guide to coin door restoration, but hopefully a helpful reference nonetheless.
To follow will be a step by step pictorial of disassembly, repair, and reassembly of both Classic Bally and Classic Stern Coin Doors. We will start with a Bally Coin Door -- I will add pictures/details for the Stern Coin Door after the Bally.
As a disclaimer my methods are not necessarily a preferred approach or the industry standard -- just the way I do things.
So lets start with a Bally Coin Door. Like many this door is a little rusty and crusty. It seems the center coin option was never installed -- I thought it may have been hacked at some point in its life but there is actually a factory looking plate blocking the coin opening above the return chute. Game test/audit switch appears to be missing, nice electrical tape job over the central lamp too. Coin mechs had already been removed upon me acquiring the door.
The chocolate shake is on standby should refreshment be needed.