I love making my warjacks look like they have been in a battle or two. While there are lots of ways to make your 'jacks look damaged, I think this is about the easiest. 'Damaging' your 'jacks is a good way to add some visual interest to your models.
Step 1: Using a dark brown (Dryad Bark here) I add linear and organic scars all over the models. I put a lot on the Juggernauts arm because I figure he uses it to block a lot of attacks.
Step 2: Add in metal colors inside about 1/3 of your scars. I am using Administratum Grey here, but you could also use a silver metallic paint.
Step 3: Add a rust color into the remaining scars to simulate older battle damage. I am using Deathclaw Brown here.