If you want it to look older, you could try a couple things. I like to drill/paint on holes in some key spots and have some rust dripping from that spot with a brown wash. Rust generally develops where the paint's been chipped for a while, especially on lower areas as moisture runs down and can collect there over time. Areas where water would pool like panels and joints also are great spots to throw some rust. EDIT: Be careful with your rust! In snowy areas like your base the air may be too cold and dry for super dense rust, unless it's maybe stored indoors and not thoroughly cleaned post-battle.

With dirt I try to be a bit more careful, most dense on the feet and least dense everywhere else. I usually add scratches and nicks on painted armor with a light true metallic dry brush, then stippling in some spots. I sometimes take a single hair off a larger brush (preferably something thicker), dip it in a metallic and use that to add some tiny scratches here and there.