I saw some tutorials on using either can or airbrush in three colours to get shading for point source effect.
Using black, grey and white.. then over coating in a very thin base colour several times...(which is best done with your own mix in an airbrush admittedly) then hand painting base detail colours then dipping with a home made mix of brown dye and mattmedium mixed version of devlan mud. followed by quick dry brushing ... worked well and looked great.
The trick there was the the home made Devlan mud has a matt medium in it and so the matt spray isn't really needed.
Perhaps you could try that instead?
The recipie is:
"devlan mud" style wash is:
3 drops elmer's (white glue)
10 drops matte medium
(this is tad expensive being about NZ$28 a 250ml pot)
5 drops water
10 drops brown ink
(Brown ink is really hard to find at a decent price.. I use water soluble powder dye now instead. same result NZ$12 for 30 grams)
1 drop black ink
Advantages are.. you can make heaps of it... use those empty paint pots