In trying to prove that Mr Surfacer is not meant to be airbrushed, I ran across a link in an old thread here of Swanny's on the subject of how to use Mr Surfacer. Here is the link, and be aware it is a very old article.
And I guess I was wrong about airbrushing Mr Surfacer, as Swanny talks about doing so. (Can't resist saying that it's not going through my airbrush though, it is gritty and to me it would be akin to thinning fine sandblasting media to run through my a/b. Uh uh.)
He also mentioned thinning with lacquer thinner as OMG did above, discrediting my second comment which was gong to be that Mr Surfacer should be thinned with Mr Levelling Thinner.
I'm pretty much 0 for 2 today so far!
....but I do hope the you find the old article somehow helpful.