Ondra, this is just a beautiful build, and if it weren't for your hand in the pics, I'd think it was 1/72nd at the least.
Don't worry about the subtlety of the rust effects, I believe less is more. After having been an armor crewman operating tanks that stayed out in the open for decades, I'm thinking these tanks probably didn't get as rusty as some would believe, so you're in great shape there.
WD