I have been using inkjet water-slide decals, with moderate success, to apply my AMA number to my planes. I am sure that there is a better way, which will produce much better results. I have looked on this forum for a method of application, but have not seen a procedure defined with definitive steps and directions that would produce the desired results. Really need some help.
Over Monokote, vinyl graphics with "top sheet" is probably the best way to go, it's the same as the sign makers use (and that's where you get them). It has two sides, a backing paper that protects the adhesive and a top sheet that holds them from the front to keep them in alignment. Clean everything up, mark the location, peel the backing and roll the graphics down, rub them, then pull the top sheet off. Perfect.
Over paint, it's the same sort of thing, except it's its a stencil, the "negative" of the lettering you want, on a low-tack paint masking material. Apply as above, pull off the top sheet, spray the color of the letters, and then pull the mask up, and you have perfect lettering.
You get them from David Powers at Aerographics, or a number of other local suppliers. The Problem Child also does it, judge for yourself whether or not you want to get involved with him.
Brett