Looks like the hardware is rusting and staining the wood. I would take the Skuut apart and deal with each individual piece. OA on the wood and Turtle Wax's Chrome Polish and rust remover on the Chrome. I can't tell what's going on with the center of the rim, but it may look worse than the edges of the rim so it might be more involved cleaning up the rust. The bolts look like a socket head and if the are a standard hex socket head then I'd replace them and the nuts with stainless steel pieces. OA should clean up the stains on the wood.
Oxalic acid
Iron, in the form of nails, hardware, or even bits of steel wool, often leaves a blackish stain on woods high in tannin, like oak. A wash of oxalic solution removes these stains as well as the grayed color of oxidized wood.
Oxalic acid is sold in most hardware stores and home centers as a dry, white crystalline powder. The crystals are toxic and irritating to mucous membranes, so wear goggles and a dust mask when handling the dry powder. In a glass or plastic container, dissolve an ounce of oxalic acid into a pint of warm water.
Make certain that you have removed all the offending metal before you bleach the wood. Sometimes stains are caused by broken- off nails or bits of fencing that are hidden in the wood. Wet the surface with the oxalic acid mixture and let it dry. Repeat if the stain is not completely gone. Once dry, sponge the wood with plenty of clean water to remove the crystalline residue. Any oxalic acid residue left in the wood will make irritating dust when you sand, so wear a dust mask and eye protection.