Why has my new timber gone blue-grey with stains?
Telling sapstain from surface mould from actual decay, which of the three you can ignore, and what to do about the two you cannot.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 4 min
Short answer
A diffuse blue-grey or black discolouration soaked into the timber is sapstain, a fungus that colours the sapwood and does not weaken it — cosmetic only, and normal on stacked softwood. Fuzzy black or green growth sitting on the surface is mould, which wipes or brushes off and means the stack is too damp. Timber that is soft, spongy or crumbles under a screwdriver is decay, and that piece is scrap.
Timber arriving from a merchant with blue or black marking on it is very common and is nearly always harmless, but the three things it can be have very different consequences. The distinction is easy to make with a screwdriver and thirty seconds.
Step by step
- Look at whether the colour is in the wood or on it.Sapstain is inside the timber — plane or sand a corner and the colour goes down into the wood. Mould sits on the surface and comes off on a cloth or a stiff brush, leaving clean timber behind.
- Test the timber's strength with a screwdriver.Push a screwdriver point firmly into the stained area. Sound timber resists and the point makes a small dent. Decayed timber gives way, goes in easily and lifts fibres in soft crumbly strings. That is the test that separates a cosmetic problem from a scrap piece.
- Ignore sapstain unless the timber will be clear finished.Sapstain fungi feed on the sugars in the sapwood, not on the cell walls, so the strength is unaffected. Structurally and for painted work it does not matter. Under a stain or a varnish it will show, and it cannot be sanded out because it goes right through the sapwood.
- Deal with surface mould by drying the stack.Brush it off outdoors wearing a mask, then restack the timber with sticks between layers so air reaches every face, off the floor, out of the rain and out of direct sun. Mould appears because the timber is wet and the air is still — fix that and it stops.
- Cut out or reject anything soft.Decay does not stop when the timber dries, it just becomes dormant, and the strength already lost does not come back. Cut well past the affected area into demonstrably sound timber, or reject the piece.
- Check what else in the stack is affected.If one board is decayed, the stack has been wet for a long time. Test the neighbours, and look at what the stack was standing on and whether water was running onto it.
- Raise it with the merchant if it arrived that way.Sapstain on ordinary carcassing is normal and not a defect. Actual decay, or heavy mould on stock you are paying a joinery-grade price for, is worth a phone call before you use it.
Tips
- Sapstain only affects sapwood, so a board with a wide pale band will show it in that band and not in the heartwood — which is a useful way to see how much sapwood a board has.
- If you are going to paint, a stain-blocking primer will stop even heavy sapstain showing through. Ordinary emulsion or undercoat will not.
- Black spotting specifically on treated timber is often just the treatment reacting with iron in a fixing or a bit of grit, and is cosmetic.
If it doesn't work
Blue-grey haze soaked into the pale part of the boards
Cause: Sapstain fungus, present since before the timber was dried — Fix: Use it. It is cosmetic. If it will be clear finished, either accept it, cut round it, or return the pack and buy a grade that excludes it.
Fuzzy black or green growth that wipes off
Cause: Surface mould from damp storage with no air movement — Fix: Brush off outdoors with a mask on, restack on bearers with sticks between layers so air circulates, and get it out of the wet.
Dark patches that are soft under a screwdriver
Cause: Fungal decay — the timber has been wet for a long period — Fix: Cut back to hard sound timber or discard the piece. Do not use it structurally, and find out why the stack was that wet.
Black streaks radiating from every screw and nail in oak
Cause: Tannin reacting with iron, not a fungus at all — Fix: A different problem entirely — replace the fixings with stainless and treat the staining. It does not affect the timber's strength.
Questions people ask
Is sapstain the same as mould?
They are both fungi but they behave differently. Sapstain grows within the sapwood and colours it permanently without digesting the structure; surface moulds grow on the outside and come off. Neither is decay, which is a third group of fungi that digests the cell walls and destroys strength.
Can I bleach it out?
Oxalic acid or a two-part wood bleach will lighten sapstain on a surface, but the colour goes right through the sapwood, so any subsequent planing or sanding brings it back. It is not worth the effort except on a small visible piece.
Does it mean the timber will rot?
No. Sapstain fungi and decay fungi are different organisms. What sapstain does tell you is that the timber was wet and warm at some point, which is worth knowing, but stained timber kept dry has a normal life.