Ink wash

Ink wash is a common medium used in modelling to create the effects of shadows or ingrained grime. The idea is that the wash fills hollows and contours, leaving the base colour showing through in any raised sections. Examples would be weathering brickwork, ageing ballast or enhancing tree bark.

Base
Per the source I washed out a half pint polyethylene milk container and mixed a total of 250ml of the clear base using the following recipe:


 * One part Johnson Klear acrylic floor treatment
 * Four parts water

Shake well, let it stand for an hour or so and it's ready to use.

Pigment
Acrylic paint mixes well with the base, you need very little to create the effect, you'll want to experiment because some washes need to be less translucent than others. Use a mixing palette, fill one dimple with the base, put a spot of paint in another, dip the washing brush in the pigment and then stir into the base. Test a small area first and add more pigment if necessary. A pipette is the best tool for filling the palette.

And it works! The result is every bit as good as the £3.50-per-pot Games Workshop product I was using before.

Source

 * http://www.paintingclinic.com/clinic/guestarticles/magicwash.htm