Black spots on red oak after being wet?

[Michael Yadfar]

I have a project I’m working on made out of red oak. After giving a good sanding, I wet the whole thing to open up the pores before final sanding. After it dried, there were a bunch of black spots embedded in the wood that weren’t there before. I’ve never had this happen to me ever, and don’t know what it is. I bought the material rough sawn so any foreign material on the outside should be gone. I used Gator sanding discs and Norton sandpaper on it, which I don’t believe have anything in them to cause this. There was no foreign material present on the rag I wet it with. The black spots sand off, but not easily.

My main concern is if this will come back when I apply finish (Waterlox). Also I’m just curious what it may be. Like I said I’ve used the same equipment and processes and have never had this happen. Btw, the camera used for the pictures stinks, the horizontal lines are from the light.

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[Doug Hepler]

Michael

Iron + oak + water suggests a chemical reaction to form a compound of tannic acid from the oak and iron. (ferric tannate?) Where the iron came from is your guess. Steel wool? Particles from tooling? If you can remove the spots you should be OK. Skip the grain raising step and seal the surface with one or two coats of dewaxed shellac. Use an oil based varnish

Doug

[Jason Roehl]

The iron more than likely came from the water itself. To dampen the oak for grain-raising prior to sanding, use distilled water. But, I'm with Doug--use shellac to seal it instead of grain-raising with water. Then you can use any film finish.

[Wayne Lomman]

Tannins do this commonly but yours is a bit spectacular. The big stain looks like a perfect match for a wet rag mark. If you had applied stain the same way, you would have had the same pattern in the finished colour. Oaks especially will turn your hands black when working with them in summer with sweaty hands.

Forget wetting and grain raising. You can see what it can do. All it does is make twice as much work sanding and doesn't improve the job. Sand it up again dry and then get on with coating. Your finish will be fine. It's a coating, not a drenching in water which is quite a difference. I'm not a fan of shellac seal coats but you will get plenty of support for their use. Cheers