I don't really remember what the thickness was. It was probably a good 2mm thick I think. It really wasn't bad, although I do wish I could have dulled it down somewhat without scratching it up and hiding the detailwork from the reverse side. As far as scratching goes, I didn't really abuse it but the thing never seemed to pick up any scratches, even with metal models moving over it and occasionally keeling over. Come to think of it, my brother's Ork ships had metal washers glued to the undersides of their bases to keep them from falling over and I don't think that even did much damage at all to the boards. The one big caveat is that they really don't travel well. One snapped when my parents changed houses, and the other one my father's using (playing surface side down) on top of some padding as an exercise mat now, and he still hasn't scratched my seal-coat on the painted side. I might have to make up another set of them once I've finally settled down somewhere and have enough space to set up a game room again.
If you decide to go the plexiglass route, get some sawhorses and a couple of 2x4s to support each panel when you paint them. These things like to sag unless fully supported and since you DEFINITELY have to paint & spray them outside, if it's hot out, you might manage to do some damage while leaving them out to cure if you don't have them well-supported. Also, as some advice, for a cool effect, pull the protective cling-wrap off the painting side right before you start toothbrush-spattering paint on. The lingering static charge will actually affect how the tiny paint droplets fall and pull some of them into some neat bands and sweeping arcs of denser "star" clusters when all's said and done.