One of the first things you do when you take a new jeep into a battlefield situation is to wash the thing with ammonia to strip all the wax and shine from it. The next is to dump dirt on it. I disagree with rust. Iron rusts within a few hundred years. A centuries old battlemech would completely rust away unless made of stronger stuff. Inconel, hastalloy and other superalloys with nickel oxidize green or pale blue. Use that color instead of orange/brown. A few good gouges across the green showing bare metal underneath and some dings in the housing would go far towards the battleworn effect.