was messing around with this knife i bought but never used and figured its gotta be good for something so when i decided to do the fat whites on the roadster i found that this knife was really good for cutting circles.
you can find them in drafting stores or the knife aisle in hobby lobby
you need all this
take the template and line it up to the size you think will look the best
lay some tape down and cut the circle the blade follows the edge perfectly without catching
lay the large piece of tape over the tire like this
mix up your paint
airbrush your tire
and youre done
before
after
if you wanna make a white wall cut another circle a size down and lay it between the big circle.