So, I just finished painting the 5 1/2" shelves with Quickcrete 1 Part Epoxy Paint... it's an anti-everything epoxy paint specifically made for concrete... but it was cheap, available, and looks like it did a fine job on the particle board. It took 2 coats, with 24 hours in between to get solid coverage, but the edges and un-surfaced back soaked it up without much expansion. Got a little of the warping you'd expect with unsurfaced particle board, but I painted the backs first so the warping would cup the bottom, which flattens out under load.
On the whole, I'm happy with the paint. If you wanted a smoother finish than I got with the roller, (a little rough), I'd suggest spraying it on instead.
