I have a great trick to paint whitewalls. It's old, but works really well.
Go to a architectural supply store and get a circle template.
There are plastic sheets with dozens of sizes of circles, that are used to hand draw at art schools as well.
You just have to pick the circle that is more correct for the tire you have, tape the others to avoid overspray, carefully tape the tire tight to the template from behind, leaving it centered with the circle you chose and just airbrush some acrylic paint on the tire in mist coats.
Man, that works like a charm!!! No brush marks, no excess paint covering the lettering on the tires, and a perfect edge on the whitewall.
This is the circle template I use more often (I have two, one in inches and one in millimeters):
Edited by Lovefordgalaxie, 23 February 2014 - 09:29 AM.