Whatever it takes to get them squeaky clean and down to bare base metal.
To do the initial clean/degrease/remove paint, etc. I use bulk carburetor cleaner, the kind that comes in a gallon can (or larger) and includes a dipping basket.
For derusting, I use a vibratory tumbler w/rust cutting media. It takes a while, 12-36 hours, depending on how rusty the parts are, but there's ZERO manual labor involved
, and the parts come out great. If there's tight spots that the tumbling doesn't get in to, then beadblasting gets the rest of it.
For heavily rusted, or large items, I use electrolysis, with washing soda. Works great.
An acid pickle, just before plating, takes care of any minute rust.
When it comes to plating the parts, I've devised a crude "plating barrel". It needs improvement still, and a motor to make it "plug-n-play", but it works and is a lot faster/easier than trying to wire up hundreds of individual parts:
DIY plating Barrel
Sean