We feed our version to astronauts.
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But seriously, I've often envisioned a cheapo (or used) swimming pool liner filled with vinegar or one of the more inexpensive liquids underneath a complete car. It would be pulled up around the edges by wires. Car would sit on bare rims to get it as close to the ground as possible; drivetrain, carpet, etc. removed. You'd probably need about 12" in depth to de-rust a subframe, floors and so forth. Maybe throw in some non-porus junk to take up volume. You'd still need a few hundred gallons I'd think.
Problem would be neutralizing it I suppose. Who's gonna try it? Javier? Sounds like a good project for an architect.