I rinsed some oak spoons in tap water to raise grain, and while drying they developed a stain within the grain. The google-box says high iron tap water can be the culprit, And another check showed oxalic acid could take them out. the cleaner ‘bar keepers friend’ has oxalic acid as an ingredient. Couple big tablespoons in 2 cups of distilled water, soaked for less than two minutes and the stains were gone, rinsed with more distilled water, and they came up awesome, and don’t look bleached. Please use at your own risk, i have no way to know what ratio / concentration i actually had, but i liked the results.
See pic with scrap / stains, and finished spoon drying after rinsing.
Might be old information to some, but figured i would share.