Everyone on this board should be intimately familiar.
I thought we could try to aggregate solutions.

Sawsall
Torch
Blasting
Chemicals
Vibratory

You get the idea

1st question: Apple Cider Vinegar ? Cheap readily available, effective?
What % (strength) is necessary? Pitfalls?

ACV - works well for me. Cheap and non toxic. Doesn’t bother my wife if a bucket sits in the garage. You can use full strength or dilute with water. I just drop the parts in a bucket and let sit for a couple days or longer if needed. Wash with water and then paint, powder coat, ect. The bare metal will rust very quickly so spray with wd-40 if you are going to let sit. It worked great for cleaning out my motorcycle tanks and getting parts ready for plating.

Does it do anything to Alum?

Not sure about aluminum but it will eat pot metal. I tossed a rusted fog light into my bucket of vinegar and the mounting bracket dissolved.

Acid is used as a pretreatment for plating. It opens up pours and cleans well. Just don’t leave in too long.

How about rollback?!

That uses the same mechanism as phosphoric acid. The phosphoric acid leaves a bit of iron phosphate on the metal, so there is a slight delay in rusting after treatment.

Apple cider vinegar is also good to treat muscle cramps. A teaspoon in a glass of water.

ACV kills warts too. Put a little under a bandaid and it turns black within day.

Eric at PMB Performance told me he uses Ketchup to clean fuse blocks. It has vinegar in it. I took the block and put it in white vinegar and soaked it a little and then brushed it with a medium brush washed it off and blow-dried with an air gun.

Used a contact spray for relays. I use a tiger grit wheel for some stuff with 220 grit in the bristles for bolts and such.

Bob B
Click to view attachment

Yep. White vinager. A couple hours and then scrub clean with an old tooth brush, soap and water