I´ve used various foams on some of my projects (mostly body panels), most seem to absorb water to one degree or another (and smell pretty bad after awhile). And if you ever do have to weld, it´s really a mess, it lights on fire and produces toxic fumes. Been there done that, no more.
I bought an undercoating gun, with a three foot extension (fairly stiff plastic tube) for spraying inside things ( I made an even longer extension). I use compressed air and the undercoating gun as kind of a solvent sprayer, except I spray Simple Green. The soap soaks into the mud and loosen things up and them can be washed out with a garden hose or high presure cleaner.
I´ve used bio-degradable chain saw oil and boiled Lindseed oil as a rust inhibitor. The boiled Linseed oil lasts for years and gets into every crack and crevice. A caution, any kind of oil in a vapor or mist is explosive. After the boiled Linseed oil has dried, it´s as hard to get out of your clothes, as undercoating or paint.
