Bruce,
You need to lay the adhesive with a caulking gun in a series of Ss. That way, air can get in from at least one direction to allow it to cure correctly. I can explain in more detail, but if you think about it like making a maze where there’s always a way out, then there will be a way for the air to get in.
BTW, are you talking about large area of plywood with foam bonded over the top? That does make it hard to run wiring, do switch machines etc. Better to build a good framework, then lay the foam on the crosspieces with a bead of adhesive to bond things.That’s a skinny area where air will get to the adhesive easily to cure it, too.
Or you can just leave the foam base loose and build your scenery as a series of liftouts, which is what I did on my Cascade Branch:
http://cs.trains.com/mrr/f/88/t/219241.aspx?sort=ASC&pi314=1