Hi Ron,
Dry brushing is a technique used extensively by military modelers I’m sure Vinnie can give you directions!
The technique is used is to dip the brush in the paint and then wipe it off ! ! A good indication of weather (!) you have it right is to draw the brush across your finger tip – it should bring out your fingerprint perfectly. You can then apply to the model, do not expect a dramatic change – you can build up gradually to get the perfect result.
A perfect illustration is to brush the edge of a metal surface with aluminium colour (silver is too bright). Thus representing where the paint has worn from perhaps foot traffic.
Another technique used is washes. To do this you need a drop of black/grey/brown (depending on the effect desired). Add large amounts of thinner until youre looking at tinted thinners. This is then painted liberaly across the surface – the colour collects on the corners / edges leaving a stain.
From experience I would use oil based paints (humbrol). The reason being if you have sealed / varnished the surface and youre not happy with the effect it can be washed off with thinners. I find acrylics dry too quickly.
Dont try and do it all at once – build it up in layers.
Hope this has been some help
Regards Kevin