Ed,
I've heard many times that you can thin Tamiya acrylics with water, but I've never used water before.
I always use Isopropyl alcohol, and always get perfect results. It flashes off (evaporates) better than water - sorry I can't give you more info.......
I've used Tamiya thinner before, but I think the alcohol works better, and it's much cheaper.
Cheers,
LeeTree