Eco-Friendly Enamel Painting?

The paint *IS* drying on the brush before it gets to the model. That's one of the drawbacks of hand brushing acrylics.

Two things you can try:
1) The type of thinner used will dramatically affect the drying time. Thinners that evaporate quickly, such as alcohol, allow the paint to dry much quicker. Water is sometimes better, but for Tamiya paints I recommend their thinner. I normally use 90% isopropyl alky to thin for airbrushing, but if I have to brush paint it (which is only for detailing) I use Tamiya's thinner. It is primarily alcohol, I think, but appears to contain some retarder since it will realls slow down the drying time.

2) Pour a little paint out (don't paint straight from the bottle since the whole bottle is drying up the whole time it's open!) and add a couple of drops of acrylic retarder. It is available at most art supply stores and will slow the drying time significantly without thinning the paint very much.