Bill Taylor replies: Just a suspicion that your customer may have supplied you with oak and not maple. The tannins in damp oak will react with iron and create a dark stain. If you have ever split oak firewood you will have seen this where the wedge contacted the wood. I have zero experience with turning wet anything so if it is maple I would rough it wet, dry it and then finish the project. Hopefully in the second process any staining would be removed. Wood is generally acidic to various degrees.
Jeff Shepard replies: I recently dealt with this on a piece of green apple wood that I turned a bowl out of. I had black iron stains that appeared all throughout the bowl. The tannins in the apple wood are very reactive to iron and cause black stains. Some of the stains were from the chuck jaws and some were from what I can only assume was metal dust from my gouges after sharpening.
I was able to get rid of the stains completely with one treatment of Bar Keepers Friend. I made sure the bowl was good and dry first and then wet the surface of the bowl with water and sprinkled on some Bar Keepers Friend and scrubbed it in with a toothbrush to evenly coat the entire bowl. Let the Bar Keepers Friend sit for about a half hour and then rinse it completely off and let the bowl dry. This completely eliminated the black iron stains and had no effect on the natural color of the wood.
The main ingredient in Bar Keepers Friend is oxalic acid and this is the key to removing the stains. You can use pure oxalic acid but you need to dilute it down so it doesn’t bleach the wood. I don’t know the exact dilution formula so I found it easier to use Bar Keepers Friend. Hope this helps someone else with the same problem.