I have been using Vallejo's Airbrush Flow Improver with my brush-painted Tamiya acrylics and it works extremely well. I usually mix a couple of drops with a big drop of the Tamiya in one cup on my pallette, and put a little of just the flow retarder in the cup next to it. Then I dip the brush in the retarder first, and then load it with the paint/retarder mix. Flows really nicely that way and stays brushable for much more than the few seconds that Tamiya normally allows. That same retarder also works with the Vallejo Model Color and MRP Aqua paints that I also brush-paint with.