Neo, sorry I didn't take before pics of the coin door. I should start doing that. I live near a industrial metal plating shop. They specialize in zinc and nickel plating. In 2009 a year after entering the hobby, I decided to restore a Xenon. I knew about the plating shop and I went in and asked the owner about replating. What it would take to make my parts look new,etc. If the metal isn't rusted or pitted, it can be dipped in a zinc stripping bath and re-plated. Old oxidation is removed and the parts look almost new again. Nickel plated parts need to be soaked for a week to remove original plating. Zinc only takes a few minutes to strip. Some parts that get really cruddy, such as drop target banks and inside cab parts need to be cleaned with acetone and sometimes a light scrub with fine steel wool or scotch brite.
As you know, Bally coin doors have that ugly inner plate. It was either galvinized or something. They don't take zinc without leaving spots all over, same goes for the big transformer mounting plate that are in early solid state Bally games where the transformer is mounted in the cab. The catch basket for the coin return is made of some cast, cheap metal and cannot be plated. After seeing some restos here on PS, pinball Mike D in particular, I saw some parts that were powder coated. I decided to do that with some of these difficult parts.
I tried bead blasting some parts to remove rust. After they were plated, they looked dull. You would think the oppsite, but blasting actually makes the surface of the metal rough, therefore a poor plating outcome. Here's a Flash Gordon I did last year...