On USAAF aircraft of WW2, what's the best way to create the wavy grey underside where it meets the olive drab upper surfaces?

I'm using ye ol' brush and Humbrol paints.

Cheers

Hi there. I've never used Humbrol paints before so I can't speak to their use. I do recommend next time you buy paints trying Testor's Model Master Acryl paints and or Polly Scale Acrylic paints. Both are easy to brush and give a good finish.

Since you are brushing, just get a good brush and draw on the outline of where you want the grey camoflauge to go. Then just fill in the inside area. Aircraft were painted with large "airbrushes" and more often than not a "soft" edge was sprayed with these paint guns.
Replicate it the best you can with a brush by following the above procedure. Although there are always several ways to do something so someone else may offer you a suggestion as well.

Maybe next time you can try using a spray can and compare the results you get between the can and the brush. Take care. Happy modeling. Russell

P.S. Always do a little bit of research before painting. For instance you can find instances where Spitfire camoflauge, at least the second color on top surface was soft sprayed and not hard masked. On the other hand sometimes it was hard masked it just would depend. The reasearch can be half the fun of building. Don't let it take to much of your time away from building though. All the best.

you can try a piece of masking tape cut to the pattern you want with a razor blade just be sure to rub the tape down good so no bleed under will happen and once dry you should have the pattern you want.