Do you mean the X/XF acrylics, or one of Tamiya's other products?
I use the X/XF acrylics, and I found over a process of trial and error that I get my best results thinning with Tamiya's proprietary acrylic thinner, X-20A, for airbrushing or hand-brushing. For airbrushing, I have used a lacquer thinner (Rustoleum's product, available at an automobile supply store).
For airbrushing, I thin to a ratio of 1:1, or a little less thinner.
For handbrushing, I use 2 methods. I will use a ceramic palette, put a little paint in a well in the palette, then add a drop or two of the thinner with an eyedropper. Not quite to 1:1, with less thinner than the paint. Or I will use the cap of the jar as a palette to pick up paint, dip my brush in the thinner, then touch it to the lid to pick up the color. Then I let it blend on the piece itself. This works very well for me.
When I learned that Tamiya's acrylics are meant to be thinned, for airbrushing, I first tried water, then isopropyl. But I found that they didn't work as well, for me, as the X-20A thinner.
If you are talking about some other Tamiya product, though, I can't speak to it, because I haven't used any of their other paints.
Hope that helps!
Best regards,
Brad
