Final Follow-Up:

I can't believe how damaged the treated tiles are compared to the untreated tile. All three tiles have completely dried now, and the results is shocking. I took before shots, and will take after shots later today, and figure out how to insert photos into threads.

The untreated tile dried the quickest, was warped, accepted the counter-bending and is now a usable tile once again. Yes, there was slight color running of the sidewalk grey, but it still looks fine. Here's a photo of the dried and counter-bent (ready to use again) untreated tile:

http://wwgallery.pcinfoman.com/displayi ... ?pos=-3752

The two treated tiles took forever to dry. Because one side of the cardstock was treated it coudn't breathe. The tiles warped considerably more. The cardstock street tile itself became horribly warped in a potato chip sort of way (as moisture from the foamcore soaked into the glued side of the cardstock), bending and dipping all over the place. Counter-bending has been less successful.

Here are photos of the two treated tiles, first hair spray, then matt varnish:

http://wwgallery.pcinfoman.com/displayi ... ?pos=-3751

http://wwgallery.pcinfoman.com/displayi ... ?pos=-3750

The two treated tiles are basically trashed. I was thinking of keeping them for other experiments but that wouldn't be fair (to those other tests). I could peel off the cardstock street tile and at least keep the foamcore for scrap, but it's warped, and I'm not sure I'd even want to use it for bracing. I think after I snap those photos . . . those two tiles are hitting the trash.

I'm really glad I did this destructive test.

[EDIT: Updated post to include links to after photos. With a gentle tug the treated cardstock street tiles easily separated from the foamcore, there being very little glue left intact. The tiles were crinkled and not worth salvaging. The foamcore refused to flatten after repeated attempts, and was tossed as well]