Oxalic acid will do the job. Go to the paint store. Wear goggles and vinyl or nitrile gloves.
Bill

 

I have only tried it once. From what I understand Bleach and oxalic acid will work to remove stains and dyes including tannin stains but only lightens wood color slightly (oxalic acid is better). If you want to lighten the wood color try wood bleach (A/B bleach). I think it is a mixture of sodium hydroxide and hydrogen peroxide. If you try normal laundry bleach is a little on the weak side so you may need to apply several coats. I have only used oxalic acid it to remove dark tannin stains.

Tim

 

The strongest bleaching chemical is hydrogen peroxide. Usually this chemical is used together with sodium hydroxide as the neutralize chemical.
You need the direct sunlight to get the best result.

Wood bleach in the wood finishing

Thanks

Good luck

 

Wiso's link is a good one but be very careful using two part bleach it is very dangerous and should never be mixed with other bleaches.
As bleaches go oxalic acid is good on rust or black stain removal.
Household bleach will lighten to a lesser degree and can be used to remove dyes if to much has been applied to a surface.
And two part bleach part hydrogen peroxide and part sodium hydroxide is the most powerful and most effective in lightening woods.
Always always always neutralize with baking soda any bleach and clean with water(and let dry) before trying another type of bleach.
If you don't you can cause toxic gases to form.

 

The best bleaching solution I've ever used is a two part product. I've used Klean Strip's two part bleach. As Jim said you need to neutralize after applying. Amazing stuff. "BE CAREFUL"