I'm currently working on "restoring" a '68 Javelin and a '66 Buick Skylark. (The Skylark is actually a "Craftsman" kit but that was basically an unassembled promo--does that count?) What I do is a style I call "Optimized Promo," that is, I'll remove all molding and parting lines, polish the glass, paint and detail the interior, and add the window trim and other chrome trim (with either foil or silver paint, depending on my mood and the subject). If the thing is a decent color, I prefer to polish the plastic rather than paint the body. (Both the Javelin and Skylark are polished, not painted.) I have a number of work-in-progress pics of each I could share if you think you're interested.
BTW, before anyone has a heart attack, I do NOT do this kind of work on pristine promos, just on "distressed" or damaged ones that don't have much if any collector value to start with. The yellow '66 Barracuda I posted in the Plymouth section is a good example of the "Optimized Promo" style.