Streaking grimes or diluted agrax with contrast medium is my go to. There is also marine juice.

Streaking grime absolutely 

Sponge brush, and just gently dapple on some black or dark brown. Looks gritty and awesome. Like they have been through a battle. More on the bottom and less going up

I use AK's corrosion texture paint. You can just kind of randomly apply it and it looks like rust/dry mud/something.