1. little bit of acrylic paint and windex (Mix ration 1:6) basically makes a dirty looking water.
2. Paint it on:
Make sure its heavier in deep spots where dirt would acculmalate, then wipe off and dab off in different direction till it looks the way you want it. Make sure some areas get alomost completely cleadned off:
Left side is unwashed the right is washed, subtle but effective
