Someone asked me the other day how I make rusty nails.
This is the sequence going from right to left.
Decide on the pattern and make holes in the wood with a needle.
Twist in tip of graphite pencil.
With a small round brush put a drop of watery burnt sienna into each hole.
Soften the effect using burnt sienna chalk pastel.
This is the area just above the coal chute.

Note:this method works best on raw unsealed wood to obtain that barn siding look.