One of our members - TEXAS FERN - uses bottle caps in a beautiful way. She fills them w/colored glass etc. and pours that clear epoxy (or rats - can't think of the name of it) over them. She inspired me to use them also. I went to a local bar to get a huge bag of them, and I save my beer bottles and caps also. I put in half marbles w/black silicon and sink BBs into the silicon to surround the gem. An example of the way I've used them are in my gift album. The sides of the pet grave marker, I made for a friend, show the bottoms of bottles surrounded w/the bottle caps mentioned above. BTW - welcome to the forum.

I would probly fill the back of the cap,for uniformity and so they wouldnt dent and for better adhesion. for waterproofness of a substrate cement board or wedi. even with humidity you will eventually have warping etc, but it will last a few years Id guess if you used marine plywood or something of that nature sealed well and brought in, kept in a covered area. You could also use some sort of board and skim coat it with concrete, then you wouldnt have to worry from what Ive been told, but there would be the weight issue.
Make sure whatever you use has no flex to it, it must be sturdy or your grout will pop off. I have a collection of colorful bottlecaps to use oneday. here is a pic of a very cool bottle cap mosaic.

Here is a link that might be useful: mosaic

Oh, WACKY - I remember seeing that mosaic, and how much I admired it. Absolutely beautiful.

Oh, WACKY - I remember seeing that mosaic, and how much I admired it. Absolutely beautiful.

wow, that's really neat - but they don't look like bottle caps at all in the pic they look like those little glass marble bead things they put in flower vases. How do you get them to look like that?
(I just wandered here from the garden web where I normally hang out)