It would depend on the scale, but I'm guessing you won't be going smaller than 1/48th Keith.
You can do a mottle quite siccessfully by using cotton buds. Touch only the tip into the paint, then squeegee most of the paint off onto a piece of tissue or similar - don't squeeze the cooton bud, just stipple it onto the tissue. Then, lightly dab the tip against the mosel, building up the pattern. Try this on a peiece of scrap first, so that you can get used to how much paint is laid down, and how to get the best effect.
Once the mottle has dried, if you want to 'soften' it, you can very lightly dry-brush over the top with the base colour, to diffuse the appearance slightly.
It takes a bit of parctice, but it works, and, until you've had a lot of practice and experience of hand-painting a mottle, it's an easier and better way to achieve an acceptable result.
Somewhere here, there's pics of 1/48th and 1/32nd Bf109E's of mine done this way, I think in '30 Years of Airframes' Models'.