Rust is a chemical process. Spray the inside of the chassis with an oil based spray. If the air/moisture cannot get to the bare metal, it will not rust. Simple.
Do not use undercoating, IMO.Its useless when it dries out.I saw many cars in the 70's/80's with rotten metal under the dried out tar. Use a proven oil/wax rust proofing spray.It also creeps into seams, unlike undercoating.
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