I was just trying out different ways to get a cast armour effect on my Pershing and tried this.

Lightly sand then stipple cellulose thinners onto a small area that you want to change (approx 1" x 1") with a stiff short haired brush. Keep stippling until the thinners evoporates and you end up with this-



You can go over areas to make them rougher.

H&S warning - wear a mask and do it in a well ventilated area because it'll have the same effect on your brain cells.