decals on new paint

Either will stain into new topcoat. It's the adhesive and sun/heat that does this. Mylar is usually very shiny, (chrome) where vinyl comes in a plethora of colors, and quality. Use good quality vinyl to hold up no matter if out in the sun, or indoors. 

Many sign painters had stock lettering they would tape or stick to a paper sheet mounted on an easel and trace the outer perimeter in pencil. Then the letters would be removed and the tracings would be rolled with a wheel that perforated the paper sheeting. This made a pattern that would be affixed to what was to be lettered, and the chalk bag would be slapped upon the perforations leaving an outline of the surface being painted onto. The sheeting was then removed and the surface would then be direct painted onto following the chalk lines. Usually the surface was scoured clean with "Bon-Ami" cleanser or similar to lightly abrade the surface for adhesion. "One Shot" lettering enamel was blended with tung oil and a couple of other chemicals that would lightly soften then common coatings used with/on automotive type finishes, and harden back up once dry. Very good product at the time.