Why have my screws stained the wood black around them?

The blue-black halo around a screw or nail in oak is not rust bleeding out of the fixing, and it does not clean off with a scrub. It is a chemical stain formed inside the timber itself — the same reaction that was used to make ink for centuries — and it penetrates far enough that sanding a plank back rarely removes it entirely.

Safety

What you'll need

• Oxalic acid wood bleach
• Rubber gloves and eye protection
• A synthetic brush and clean water
• Stainless or silicon bronze replacement fixings

Step by step

1. Confirm what you are looking at.Iron tannate is blue-black to grey-black, spreads outwards in the grain from the fixing, and is worst on the wetted side. Rust staining from the fixing is orange-brown and sits on the surface.
2. Know which timbers do it.Strongly: oak, sweet chestnut, western red cedar, iroko, walnut. Mildly or not at all: pine, spruce, poplar, most plantation softwoods — which is why the problem is invisible until you use a decent hardwood.
3. Remove the offending fixings first.There is no point bleaching a stain while the source of the iron is still in the hole. Take the fixings out and set them aside.
4. Mix and apply oxalic acid to the wetted area.Follow the product's instructions for dilution, and apply generously with a synthetic brush over the whole face rather than just the stained spot — treating one patch produces a pale patch instead of a dark one.
5. Let it work and repeat if needed.The stain usually fades within minutes to an hour. A second application is often needed on old deep staining. Keep the timber damp with the solution while it works.
6. Neutralise and rinse thoroughly.Rinse with plenty of clean water, and follow the maker's guidance on neutralising. Residual acid crystals in the surface will disrupt any oil, varnish or stain applied later.
7. Let it dry fully and expect grain raise.The timber will need at least a day or two to dry and will need a light sanding with fine abrasive to knock down the raised grain before finishing.
8. Refix with stainless or silicon bronze.A2 stainless is sufficient for interior and sheltered oak. A4 where it will be persistently wet or coastal. Silicon bronze is the traditional answer for boat and exterior oak work and looks appropriate on it.

Tips

• Galvanised fixings stain oak just as readily as bright steel. Zinc is not the issue — the iron underneath is, and any breach in the coating starts it.
• The same reaction is why you should never use steel wool on oak, and why a steel clamp left on a damp oak board leaves a black bar across it.
• If the timber is going to be limed, ebonised or fumed, the black staining may be irrelevant or even desirable. Establish the finish before you start bleaching.

Common mistakes

• Sanding to remove the stain — Iron tannate penetrates millimetres into the grain. You will remove a lot of material and often still see it, and on a moulding or a veneer you will not get it back.
• Bleaching a spot rather than the whole face — Oxalic acid lightens sound timber as well, so a spot treatment leaves a light halo that is just as visible as the dark one was.

Questions people ask