Hey 40.mm if I may 'chip' in (sorry about the pun) Freddie, what you're doing here is.
1). Paint model your undercoat colour, aluminum here or maybe primer red on a tank or truck.
2). Spray the model with hairspray. I use two coats, you can go with one or more depending on what works for you.
3). Airbrush model with the upper coat, aka green over grey here.
4). When the upper coat has dried, lightly scrub the model with a moist (not wet!) large stiff brush. In the high spots where the brush catches it will cause the hairspray to come up along with the paint on top of it showing the undercoat. Sorta drybrushing in reverse.
5). Coat the model in several good coats of the clear coat of your choice. A coat of gloss for decaling and then a coat of matte for the finish should be enough. If you don't and wash the model later on ALL the paint will come off...
I have no idea how 40.mm did his, I'll leave it to him to fill in the details. I use acrylic paint and I've heard it works better than with enamel but I never tried enamels here so I'm not sure.