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You can't seal or encapsulate rust. It is just like cancer, it has to be cut out. The rust will keep on eating.
I would be afraid to throw an over-nite bag in there.
My suggestion is to try and find a SOUTHWEST salvage yard that specializes in old cars and see what a cut-out with shipping will cost.
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- WHY IS IT ... 
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... that everything you buy in the grocery store is either wrapped in or contained in plastic but the government doesn't want you to carry it out in a plastic bag?
WONDERING IN W(BY GOD)V ...
DIAGNOSED CDO - (OCD In Correct Alphabetical Order)