Spraying over paint with Mr Color Leveling Thinner . . .

Thanks everyone for your input.

I'll have to do some experimenting on my paint mule.

I do this all the time when trying to achieve a bit of extra gloss or smoothness.  It doesn't take much, so be careful to avoid full thickness melting and running.   I'd also be a bit reluctant to use it on metallics.  I generally put a drop or two of Rapid Thinner in those when spraying so they set quickly.  Slowing down metallics is asking for the particulates to sink and settle, ruining the metallic effect.

Although its not quite the same thing, I have used a similar method for smoothing paint on the insides of jet intakes.  Whatever paint I used, I shoot some of the thinner I had used with that paint and it does smooth it out.

In the past, I have also done something very close to that with Model Master enamels, using mostly enamel thinner, with just a little bit of the paint mixed in as a final coat.  Gave me such a good gloss, I was afraid to touch the model because it had a perpetual "wet" look to it...even though it was fully cured.

If you mist some lacquer thinner over the finish color coat with paints that accept LT supposedly you get some extra leveling and perhaps gloss. I've never done this but I also have never heard of it being done to primer.