I hand-brush Tamiya acrylics (X/XF) all the time, and I use Tamiya's acrylic thinner, X-20A. I originally used water, and isopropyl, but the paint would clump, or lift up previous coats. Some colors were worse than others, but none of them were satisfactory. Once I started using Tamiya's proprietary thinner, those problems went away. I can lay down colors in coats as thin as if they were sprayed.
I use two methods, really depending on how I feel. One is to use a little jar of the thinner as a working supply, pick up the color with the brush, whether from the underside of the lid, or direct from the jar, then dip it in the thinner, and apply it to the piece. It levels out as I apply it.
The other method is to use an impermeable palette, like a ceramic, plastic, or glass, and mix a little bit of the paint and a couple drops of the thinner, till it's consistently mixed. Then I apply it to the piece.
I don't use anything else, no retarders or anything else. But that's just my preference, because I haven't appeared to need such adjuncts.
I would not recommend using any lacquer thinner to thin for hand-brushing, because the thinner is so hot that it's liable to lift up any color coat you already laid down. However, there are Japanese painters who use lacquers to paint by hand, particularly in the Maschinen Krieger genre. They use the fact that the thinner dissolves, or reactivates, the color already on the model, to blend the colors. I just don't want or need that effect in what I paint.
That's all based on my experience, and it works for me. Your mileage may vary, as they say.
