A method I've seen used, was with a broad, short bristle brush (like a "chip brush") with a thinned paint solution and "flicked" at a distance from the edge of a palette or something similar.
Search for "stencil"
Gary in the local IPMS group uses them for irregular fading undercoat on wings.
Slightly lighter than wing color then thin overcoat in the wing color.
A mottled effect which is quite nice.
Expensive little buggers tho.