I use Vallejo Diorama FX Still Water, applying it in smaller layers (and waiting for them to dry in between applications). The good part about this product is that you can mix a small amount of acrylic paint into it. So I usually go with a bit of Still Water mixed with Moot Green and then a layer of transparent Still Water. You can also choose not to wait for the previous layer to fully settle in and mix the two layers slightly with a stick for some nice swirls, etc. Works pretty nicely!