Gloss or satin coat if you are finding you are getting tide marks, also aids in the "flow" of the wash, as it will travel along seams and such using capillary action.
I have heard some people cutting the wash about 50/50 with water or Vallejo thinner, but I use it neat, but only use it with a very fine brush, and basically "draw" the wash along the crevices and such where you want it.