The props certainly look much better Jeff, and that mottling turned out well.
Before I got a fine-line airbrush, there were two methods I used for mottle.
1). Use a cotton bud, dipped lightly into the paint, and then wiped off, rather like using the 'dry brush' technique. This is then lightly pressed onto the model surface, and twisted, leaving an irregular, soft 'spot'. When all the mottling is done, a very light dry-brush over the top diffuses the effect, giving a passable mottle.
This can be seen on the (brush-painted) 1/48th scale Bf109E below, built around 25+ years ago.
2). With a 'non fine-line' airbrush, create a mask, using copier paper, or better still, tracing paper (this allows at least some 'view' through the paper, allowing better alignment.). Cut irregular holes into the paper, the size of which (the mask) should be small enough to allow it to 'fit' to the desired area - only 4 to 6 holes are required. Hold the mask clear of the surface, and spray through the holes, repeating this along the area to be mottled. Some trial and error is needed, to establish the required distance of the mask from the surface, and the distance of the airbrush from the mask, and practice on a 'hack' model should sort this. Also, thinner paint, and a lower pressure is desirable.
There are, in fact, similar masks commercially available, but whatever the source, one of the problems of this type of mask is actually placing and holding it, although they can often be lightly taped in place, using a card 'spacer' to lift the mask clear of the surface by the required distance.