The McMillan stock company uses Polyurathane paint on thier stocks. Most others use a similar product, utilizing several coats. I haven't looked yet, but I plan on camo-ing my own stock soon. I'll post after I find something. If you find out anything, post us as well.
I've found when PAINTING wood it gets a little tricky! For example, if you use a spray paint then put polyurithane over it, the poly. will eventually bubble and flake off! Now when painting I use a clear laquer in a spray can. (the high temp clear for engines is'nt supposed to yellow!) For a camo job I'd go to WAL-MART, and get a cheap TESTORS airbrush. AS far as paint for camo, acyrilic is cheap and some brands are water resistant. Myself, I'd clearcoat this also!