I just finished painting the front of my back wall pieces.
A note on the foam: it doesn't just expand, it also contracts several hours later. If you are going to use a thin media for your back ground (like I did), you will need to weigh it down with rocks once it is dry enough. For most of it, I had enough rocks in the mix to do it, but I had to train a few bits to lie flat.
I used the Sweetwater Epoxy paint -- let me tell you, do NOT get this on your hands, or the floor, or your counters, or anything else. It's a task to get up! Part A melted the plastic cup I was using to measure, but fortunately not my nitrile gloves -- at least not once mixed.
The fumes are potent. Take it outdoors. It's a bit thick and difficult to get in all the crevices -- a flat brush like a stencilling brish will help. I went ahead and painted the rocks, too, since I figured water would seep through the rock and break down the foam anyway if I didn't.