A friend of mine showed me a card he made as a part of a gift for a relative, (re-producing a childhood card game with meaningful photos of family, etc.)
and I though the card seemed quite nice.
His method: Print out the fronts on photopaper, and the backs are shelf liner paper, or mac-tac. Basically that gluey sticker stuff that goes in the bottom of shelves. It comes in basic patterns and colours, and although his was a purple-marble-ish pattern, I bet there's some plain ones out there you could even print a back onto.
I like that it wasn't laminated, as I find that a bit too thick, and it did have a surprising amount of stiffness. I assume you'd want a more matte photopaper as opposed to a really glossy one.
Possible flaws? Maybe with lots of shuffling it would start to peel at the corners. Maybe the photopaper side would get scratched a lot. I think my friend and his family might play a few games, but probably not any more than that, so I won't have his info on how they hold up.
Overall, I think it looks like a pretty good idea for prototype cards anyway.
He showed me one,
Actually the only reason he showed me was because he happened to have one on me and we were talking photoshop. He was proud of his accomplishment with the photo, but I was impressed with the card itself.
I'll ask him about the gumming.
There was no curling, and the edges weren't sticky. It really seemed the expected drawbacks weren't present. That's the main reason I bring this up.
Also, what is this spray plastic you speak of? (Maybe he used some... I don't know.)