Andy, in 1/72nd and 1/48th scales, I used to use a cotton bud (Q Tip?) for mottling, and it works quite well.
Here's how to go about it.
First, paint the base colour of the large area, eg., RLM 65 Hellblau, as normal.
Next, take some cotton buds, and pull-off any loose fibres, then smooth out with your fingers.
Dip the tip only into a vey thin layer of paint in a pallete, and wipe off the excess by 'dabbing' onto a piedce of plain paper (not tissue).
Gently and carefully apply each mottle spot, in a sort of dry brush method, and give the cotton bud a gentle, light twist at the end of each application.
When all of the mottling has dried, go over the entire area, using a dry-brush, with the base colour (RLM65 in this example), to diffuse the edges of the mottle. Do this very lightly, so as not to detract from the colour(s) of the mottle itself.
This should 'soften' the edges of the mottle, and slightly tone-down what will at first appear to be too heavy a colour for each mottle spot.
When the clear coat is applied, the mottle will appear to jump out, and may look a little too stark, but once matt/semi-matt coated, the diffusion takes place again, and the job should look quite convincing.
I used this method for over thirty years, until I got an airbrush that could cope with smaller mottles, and some of the models done using this method I had to look at twice, in order to convince myself they hadn't been sprayed. It also works very well on armoured vehicles, for some of the German colour schemes, where a brush might be too 'heavy'.