Jeff,
I do all mottling and squiggles freehand, which is the way the originals were painted (with a few exceptions, like very late war G/Ks tails). I’ve seen a few models painted with masks but to my taste it doesn’t look as good. Might be relevant to say that I only do 1/48, a scale that makes this kind of airbrushing more “friendly”, but there’s a lot of talented modelers here that do it in 1/72 with precision.
Here’s one with a rather heavy mottling
