I don't claim to be an expert, this is a question in general. . Primer is one thing, if you get a scratch or little discolor all you have to do is lightly sand it and or spray some more over the spot. But with gloss you can buff small scratches and water spots out like it never happen. If you got a little scratch or whatever on a vehicle with a flat paint job, how would you fix it without repainting the whole thing over again. From what I've heard is that flat paint is like painting with candy. It looks good new but if you have to repair one spot, you have to be real good to get a good match to the rest of the vehicle.