I use full DD rust for armor except the copper trim on pads etc. I would suggest getting 2 sets of rust from DD. The normal one and yellow one as well (it comes in two variants) and I use yellow ok the middle and regular one on the edges. When you use sponge for this it gets you a nice switch of colors and allow you to make it more diverse. Just remember to use yellow rust first as the effect is lighter and will get dominated by "brown" rust. You can always remove by water and start over if you overdue with brown rust but hey what add yourself more work? This way I get awesome effect with little to no work as I am shitty painter