Are you looking for rust, oil stains or battle damage?
For rust I use a base coat of Scorch brown, stippling of Dark Flesh, a light stippling of Blazing Orange and a very light stippling of Bolt Gun Metal. After that I dry brush Blood Red over the middle of the panel. Here is a copy of pictures from an earlier post of how I did my rust weathering.
For oil I just use
GW Badab Black wash to stain the area I want weathered.