You guys are right about the hubcap stencil.
Using masking tape may create a nice and sharp line - looks good (or so) if done properly.
However nice you might like that; factory original was the use of a stencil, not (a special) tape.
This gives you then the absolutely original faded-out shape/edge/spray pattern that the original hubcaps had !
However, I understand very well that the today available reproductions made from rubber or silicone are too bulky and too wobbly to provide a good (or authentic) paint result. The original tool was made of steel, much thiner and stiff. That's why the paint result was much better than with the today's stencils.
Ask me how I know ....
Achim
(dogdishhubcapcollector)