The dry brush technique is used to highlight edges, bolts, rivets and generally all the protruding parts of the model. This helps to achieve a better visual depth, and provides also further interesting effects.
The technique itself isn't so difficult: first, just dip a stiff paintbrush (preferably an old one) into a paint with a lighter hue than the base color. Wipe then it on an absorbing —but not too rough— surface (the best is an old newspaper) until very little of it remains; finally rub it onto the model. The few remaining paint particles will thus color the protruding details, enhancing them.
Usually slow-drying paints such oils and enamels are used for dry brush, but you can achieve fair results also with acrylics with a little bit of practice. In this case you should use a retarding medium (I used the Vallejo 70.597 Drying Retarder) in order to slow down the paint drying time and use the color mix longer.
Hull and turret
For this model I used the same mix of Dark Sea Grey, Olive Green and Buff I used to brighten the centers of the panels (you can find it in this post), trying to paint especially the edges and the protruding details without anti-slip:
The zones with anti-slip are the most troublesome for the dry brush, because the details would be highlighted as well as the texture and its grains might be removed by the brush. In this case I painted instead the bolts and the raised edges with a fine paintbrush. Actually I should have made this passage before applying the filters, in order to blend the effects, but even so the look is quite good.
At the same time, I mixed 50% Vallejo Model Color 70.991 Dark Sea Grey and 50% 70.965 Prussian Blue and I dry brushed it onto the parts that in the actual tank are made of rubber: I thus gave an impression of weathering, due to the wearing and the sun:
I also dry brushed the burnished metal parts —like the machine guns— with a steel enamel on a black base. This dry brush has been stronger on the rail of the tank commander's .30 cal. machine gun, owing to the more friction this part sustains (moreover, at the end I will rub a pencil tip on its edges, to simulate the polished metal effect).
On the tow cables (a part whose final result I didn't like too much) I made a dry brush with both steel and rust (Humbrol 113 Matt Rust enamel) colors:
Finally I sprayed a coat of matt varnish, in order to protect and seal the underlying layers. This also hindered the polished effect of the burnished metal parts, which I recovered with a wash of Mars Black oil color.
In the picture below you can appreciate the difference between before and after the dry brush.