I have used them on some resin stuff. They worked really well. If you do not seal them they will rub off and if you get them wet they reactivate and will wash away easy. Definitely seal it.
I used it on some armor and got some cool transparent effects of rusty / mold on the armor. Very cool! I hear ya about the wal-mart paint, even with the FW inks I largely use the walmart paints cuz they are just so cheap.