MihaiGeorgescu,
This would be the 1st armor piece that I have done using ALL acrylics (instead of enamels)
After the base coat has dried for 24hrs, I sprayed a gloss coat over the entire model. I then used artist oils, Black and Burnt Umber, for the wash. I mixed the oils with 100% Mineral Spirits. I first applied the black so, it would bring out the hidden details and lines (let sit for about 30 minutes) I then applied the Burnt Umber for those places I wanted to show more rusting and dirt build-up. Let this all dry for about 3 hrs.
I then took a Q-Tip (you can use any sort of cotton swab), dipped it in the Mineral Spirits and lightly began removing the unwanted wash. You have to change swabs often because, you will find cotton strings all over the place.
After the excess wash has been removed, let dry overnight. DON'T TOUCH IT. Y ou can now start drybrushing and adding more rust and highlights. Once you are satisfied with the look, give it a good shot of Dull-Coat.
Note: For the shell hits, I heated the head of a small nail and pushed it into the model. Before it gets cool, give it a twist so it doesn't look perfectly round. I then heated the other end and scratched around the inside to give it some character