In my youth, a friend of mine's dad was a Harley buff. I mean back in the fifties, and from then on. He ended up having a shop, and turning into one of our area's authorities on Harleys.
He used to have a 55 gallon barrel out back of the house a ways. It was filled with some liquid. They would hang fenders and tanks on wire and lower them into the liquid. It wasn't terribly caustic or acetic because they didn't really treat it any special way by wearing gloves or goggles.
It was up on bricks. Then would put a couple of pieces of 2x4 under it, and light a small fire. All the paint would be off in a day or two.
Anyone have any idea what they used? They were old farm boys, and really thrifty, so I know it had to be some very common cheap substance.