Before you put in the tar paper and once you have the floor exposed and clean as possible. Try using Pour 15 ( I think that is the name of it) IT converts rust into a primer base and will hopefully stop the rust and seal it before covering it up. Allow time for it to dry completly before covering it up. It is a brush on paint and is very tough, made for coating rusty areas. Check with a local auto parts that handles automotive paint and they can advise you as to where to get it.
It works great to stop existing rust if you are not ready to replace panels at this time.
Dennis
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