Hi;
I use Testors Flat Rust. Dry brushed and thinned to almost opaque! I have done that for years. The funniest model I saw and I had to Not laugh the work was so good, was a filming model for an ad. The builder was a very close friend and I did not want to hurt his feelings.
Seems he had rust all over many places you wouldn't have it on a W.W 1 Sub Chaser.( They were, at that time made entirely of wood!