...more thinking... would it be best to find a flexible adhesive...being are a fixed and flexible surfaces I am bonding...
Once installed it wouldn't have a need to move at all and therefore would not need a flexible adhesive would it? I have used Liquid Nails to stick a treated 4 X 4 to a brick wall, but thats the regular Liquid Nails stuff, you just need a good bond between the two pieces and I still think the expanding foam would do the best at it seeing it's able to "fill" in voids and it will bond pretty good to metal, I once used it as backing for bondo on an old pickup cab I had that had rusted corners, it stuck real nicely.
Well I would think as long as the metal part was clean and free of loose material, the way I would do it is shoot just enough foam on the metal part (and this is the hard part, getting just enough and not too much so it comes out the sides) and immediately place the pad over it and maybe wrap it or bungee it so it will stay tightly squeezed together, if any excess comes out just trim it off with a sharp razor blade after it dries.
Liquid nails has little or no flex when its dry. What about gorilla glue. Ive never used it, but alot of people swear by it.
I use the stuff quite often to repair the floors of fiberglass bathtubs and other projects. What I would do is to QUICKLY apply a thin layer of it on the metal first with a disposable spreader.. to "knock the air out of it" and turn it into a thick mastic type glue. That will allow it to bond very well to the metal and create a good base to apply the cap onto. Then apply the foam directly to the dashpad in very quick and complete fashion without lingering in any one spot for too long to avoid excessive build.. and then slap the two pieces together and clamp them tightly to avoid further movement. Clamping them together uniformly would be key though as the foam will continue to expand and push/lift the pad off the metal frame over the first 15 minutes or so. Joe Dirts urethane idea would also work well.. but that stuff certianly ain't cheap either. Another way to go would be to bite the bullet and just buy adhesives which were meant for this very purpose. 3M.. Dap, etc.. are all good products.. some better than others. Here's another which seems entirely appropriate for that project. http://www.perfectfit.com/24783/247263/Upholstery-Adhesives/VS-Foam-and-Fabric-Adhesive.html
I second the Liquid Nails. I've used it to fasten carpet to metal before back in the custom van days. It never came loose until I pulled it down years later, and even then it left carpet fibers on the metal! I think expanding foam would expand and push the pad away from the metal before it dried.
Black RTV silicon works really well. lay it on thick and once you have the pan in place, weight it down and let it dry. Sticks to both of the materials you are working with and does have some flex to it although the flex is not needed. I use landau adhesive in the shop.
thanks everyone, for staying mostly on topic...:Handshake "adhesives which were meant for this very purpose." I think I will run it by my upholstery Guy and see what he has to say...