I sometimes brushpaint cars. Since I don't have a spraybooth, I have to do all my spraywork outside. So spray painting a car body in the middle of a snowstorm is not posible. Airbrush or spraycans are by far the easiest/best way to paint a carbody, but with lots of practice and the right paint/paintbrush you can get some fairly good results with a brush.
A few month ago I had a Dodge Viper on display, witch is brushpainted.
Painting model cars with a brush, I use a flat and very soft sable. I once bought a very expensive flat W&N Kolinske sable, for painting cars, but the bristles were not soft enough, it left brushmarks. I then bought a quite inexpensive and very soft sable, which did the job.
When brushpainting modelcars, I always use Humbrol enamels. I find them superior for this particular use.
When painting large gloss areas, with a brush, I do the opposite of what I would usually do when painting. I put on a fairly thick coat of paint, rather than applying many thin coats. I don't slush the paint on, but apply a coat thick enough for the paint to level it self out, but not so thick it will start to run down the sides of the model. Always paint in the same direction, not back and forth.
You could experiment on a piece of scrap plastic.