Have you ever thought how custom painters paint designs on cars for instance on the extreems of the curves of flames ?
In the painting shops you will find a very narrow tape ~~~ buy 3M's you can be assured of no bleeding under. After you applie this narrow tape you will find it easly following along the curve of the design. then follow up with a wider version with paper attached to catch the over spray, sticking it to the edge of the former more flexable earlier version that followed the curve. Then take a scotch brite and carefull ruff up the surface, make sure to get rid of any dead paint,Use a fast drying thinner, you'll want to get to the solid surface. spray a good enamel base paint for you design, out of the sun.