I cut the Vallejo washes with their Glaze Medium which is a little different from their Thinner Medium (I also use Glaze Medium to make washes out of their paint).  The Glaze Medium does a few things, most importantly it GREATLY extends the cure time:  even if the wash looks dry, it will not have cured and can easily be tidied up with a qtip dipped in booze/alcohol (care must be taken not to overdo it and cut into the underlying paint).  This is how I get nice clean lines on my Mobile Suits.

The other thing cutting it with Graze Medium does is it "softens" the effect a bit allowing you to build up the color and feather edges, like when doing rust streaks etc.

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