How to fix watermark on cedar?

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

Hi there, apply a dab of mayonnaise with a soft cloth and rub it into the mark in small circular motions. Leave it on for an hour or two (or even overnight) and wipe and buff with a soft, clean cloth until. If the watermark is quite deep, dip a rag into a mild solvent such as mineral spirits or odorless paint thinners. Squeeze the excess out and then rub gently until the stain is gone. You will need to test that the solvent doesn’t harm the finish by putting a small amount on an inconspicuous spot (perhaps the underside). Best of luck