Bill,
I recently wrote an article for the GTD club on paneling and other operations which included a section on insulation, which covered all types of insulation from the mats to the expanders. I can send you the full article if you are interested. What I wrote was the following about the expanders.
"A cheaper more labor intensive alternative is to consider the insulating foam sealants used in the building industry. Names like Great Stuff comes in aersol cans at most building supply stores. NTAP X-30 Polyurethane Foam, is a 2 part pourable foam expander. When mixed 50/50 A/B by weight, the mixing time is 20-30 sec. with a rise time of 2-3 min. It is tack free in 3-10 min. with full cure occuring in 24 hours. This produict is best done in several small pours rather than one big one. Fomofoam, a two part spray for verticle surfaces, comes in propane type containers. All form airtight and waterproof seals, bond to most materials, cure rigid, and are easily trimmed with an electric carving knife etc(for open tube frames). Just be aware that they can be messy, as they are very sticky. If the surface or adjoining surface is a paited surface, it should be covered with "plastic wrap" from the kitchen drawer and taped in place. All holes and gaps should e taped closed unless used for the expansion of the foam. But remember this stuff is expansive and will push things our of its way if there is no place for it to go. If you have double layered panels, it is a great filler and insulator. Just give it an escape route for accidental over filling. If it does get on the aluminum or stainless, solvents/cleaners/polishes or elbow grease will bail you out. If used in the central tunnel, block off the cooling pipe holes or cover the pipes. Wrap all wires, cables, etc. in some form of PVC piping or kitchen wrap. Be sure to line the tunnel cover with the kitchen wrap so that it can be removed for repairs. Remember this stuff bonds to all surfaces.
Mixing two liquids, a polyether polyol and a polyfunctional isocyanate produces polyurethane foam. Like all organic material, when they are exposed to an ignition source with enough heat, polyurethane foam products will burn. Even the most flame retarded urethane foam will undergo combusion at temperatures exceeding 550 C(1022 F). This is a hazard to be considered. Having said that, one must remember that the fiberglass that surrounds us is also combustable."
This doesn't cover all the materials out there. Do a Google search for foam insuilation. Just take your time if you use the pourables, use a lot of tape to secure "handyWrap" on the "show" surfaces. If you use the spray type, drill holes just large enough for the spraytube to enter the void at about two inch intervals across the surface, with a larger hole at the end of the panel for escape. Be sure to give it some place to go. If you plan on putting leather or some other fabric on the panel, all will be hidden. I used it on almost all of my stainless paneling and it was a breeze. But them stainless is a "little" stiffer. Had to do a little clean up, but wasn't that difficult.
Get some thin shipping boxes from the post officeand try some of it, and you will learn real fast how much goes a long way, and how hard it pushes on the surfaces as it expands. When done you will notice that the doors will shut with a dirinte solid sound, and the interior of the car eill be much quieter. Fewer surfaces for the sound to reverberate off of.
Need more info or would like the article let me know.
Bill