Do a quick pass over your model with a satin or gloss varnish - this will protect your paint job and help the wash in the next stage...
Stage 7: Apply a wash
There are lots of ways this could be done - I'm a big fan of Citadel Shades, for example, but wanted to try something darker for this project. On large tanks like this, I like to use MIG Productions "Dark Wash".
This is an enamel wash. Simply load up your brush, and set it to the grooves and recessed parts of the model. The wash flows right off of your brush, and into the detail. This can be messy, but you will be left with something like this:
Remember to wash your brushes well after using this wash - it will ruin expensive brushes, so use something cheap for this part.
Stage 8: Clean up wash
This stage demonstrates another nice quality of the MIG enamel washes. You will need some thinner - I recommend MIG "Thinner for Washes."
Take a piece of sponge (from a blister pack is fine), and dab it into the wash. I use the inside of the thinner bottle, as the thinner evaporates really quickly. Using the sponge, wipe the exposed armour plates. This will leave the shading in the recesses, whilst removing the excess wash. A side effect is that it also stains the whole model, giving you a weathered and dirty overall look.
Next up: the finished model...