Rex, I use Mr. Surfacer 500 for most cast effects and 1000 for that pitted rolled steel look you can find on Tiger II and Panther Tanks. In general I use a sponge technique to apply the Mr. Surfacer. The sponge material has sort of a micro bubble or pit look to it. I guess a better way to discribe it is the holes in the sponge will determine how cast or pitted the surface will look. also the amount of Mr. Surface used will effect the look as well. I used an old Bandai 1/15 StuG fighting compartment to get my technique down. I have also used a stiff tooth brush to get a more pitted and scratched look by stabbing the tooth brush into the surface.
Here is the site I found that put me onto this method. The guy is using it on Gundams but the process is the same for
Armor/Tanks. First he shows a process using liquid cement and a tooth brush and then he shows the sponge process with Mr. Surfacer. I have done the liquid cement method but I like the Mr.Surfacer sponge/tooth brush process better.
http://gamerabaenre.com/?page_id=1417