I'd prime it in something that will stick, like zinzer 123 or a spray prime, and then paint it as you would anything else.  For dirtying down I try and use powder paint mixed up with water and a little of PVA to fix it.  However you could use any water based paint really once you've got a good prime.  Build up in several thin washy layers,  rather than trying to do it in one go. A raw umber and maybe a bit of Olive green, and some rust tones wood look great.   Another favourite if you want a bit of it to look wet is to run water over an area then shellac on top, then spalsh water immediatly into the wet shellac.  This make the shellac go white and congeal at the same time. It looks like years of leaking pipe when dry. A final thought is to spash meths(denatured alcohol) into the last layer of wash, just a bit.  It moves paint around for you really nicely, especially powder based washes.  I would paint with a natural sponge, as well as a brush and let gravity and water do half the work for you, but keep the sponge and any decent brushes away from the shellac.