I went through a phase of doing this on a lot of my old cars. The trick is to spray it indoors (garage etc) and get a good smooth finish. Mine looked spot on, passed various MOT's and i one masked the red reflector off and sprayed the rest of the light. It was a red car and looked smart with the red "line" running through the tint. Don't do more than a couple of coats though as there is a minimum light level for the MOT/coppers.
Oh and i sprayed the outside of the lens after sanding down the little moulding marks with 1500 wet & dry.