Apply mayonaise. Let it sit for about 15-20 mins then buff with a clean soft cloth.
You mix up a saturated solution in water and then apply the bleach to the entire stained area. It should remove just the dark spots caused by rust, water, or metal residue without affecting the surrounding wood. After the application is dry, there will be some oxalic acid crystals left on the wood.
I would see if you could lightly sand it then treat cedar with lineseed oil