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My oak has gone black

My Oak has gone black! How do I remove the staining?

Ever since I wrote the guide to painting or staining gates and doors, I’ve been emailed or phoned hundreds of times about a
problem a lot of people (not our customers, I must add) are having with their
products, be it garage doors, front doors, gates and even garden furniture
usually in Oak. It usually something like: my Oak gates or garage doors have gone black and is this normal? Why have they gone black and is there
anything I can do about it?
Black stained Oak

For example (the picture above accompanied this email):

Good evening, I wonder if you wouldn’t mind giving me some advice. I purchased
some Oak gates in 2013 and since then, I have been treating them with Cuprinol
Teak Oil every year. Over time, they have lost their colour (see photographs
attached). Does this look like natural weathering or is the wood infected with
a fungus and in need of washing with a fungicide or mild detergent? To restore
them, would I need to sand down or just apply a good quality stain? I would be
grateful for any help you can give.

Why does Oak turn black?

Oak can turn black as ferrous fixings (nails, screws, hinges, handles etc) have
been used somewhere in or on the timber; the tannic acid (this is what makes
timber like Oak so durable and long lasting) present within the Oak then reacts
with any ferrous fixings and the result is a dark bluey-black stain. This
staining is the ferrous ironmongery corroding. However, it’s not just Oak that
can be affected with this: several other timbers can produce the same problem,
most notably Douglas Fir, Teak, Ipe and Iroko. All fixings and ironmongery
should really be non-ferrous to avoid this staining; in plain speak, this means
using stainless steel or brass. As you’d imagine for things like front doors and
windows, you’ve a vast array of stainless steel and brass fixtures and fittings
available to choose from, but with gates and garage doors, you’re really limited
to just stainless steel and only really Grade 316 or A4 Stainless steel, which
is “marine-grade” stainless steel. This is, however, an excellent choice as it
is extremely corrosion-resistant, hence the marine grading. It should also be
said that the same goes for any fixings within your gates, doors or windows etc.
and not just what you can see. Any screws, nails or any other type of fixing all
need to be non-ferrous so this excludes bright zinc plated ironmongery (BZP) or
‘Self colour’ ironmongery. So if you’re reading this and about to get something
made in Oak, then it may be best to ask the question; that said, any joiner will
be using the correct fixings but not everybody out there claiming to be a joiner
is a joiner!

How can I remove black stains from Oak?

Firstly, you should try and remove and replace any ferrous fixings and
ironmongery, as once you’ve sorted out the black staining this time, it will
certainly reoccur in the future. Though if ferrous fixings have been used to put
the item together, then unless you’re lucky, you’re not going to be able to
remove these, so you’ll just have to make do and replace what you can.

What won’t work:

  • Sanding down doesn’t really get rid of the problem easily, the staining can
    go quite deep so you will have to sand a fair bit of timber, causing large
    indentations in the timber.
  • Don’t be tempted to rub down with wire wool, this stuff is ferrous and
    you’ll find you’ll leave small bits of the wire wool in the timber and won’t
    get rid of the original staining.
  • Don’t try and jet wash the timber, it will take ages to dry out and you
    won’t remove the stain.
  • Don’t be tempted just to stain over it; most wood stains are translucent, meaning you will see the staining on the timber through
    the wood stain (confusing I know!).

To show you how to get rid of the staining upon the Oak, and not having any
stained Oak to hand, I will have to make it stain!

Ta-dahhhhhh… Blue/black stained oak!

I left two pieces of Oak out overnight with several bright zinc plated (BZP)
nails upon it. I then soaked the nails and Oak with water and left it till the
following morning.
Oak water BZP nails
By the next morning, I had this to work with! * The staining will not usually
happen quite as quickly as this, it’s been quite fast as it was drenched in
water and the nails were BZP; if galvanised was used instead of BZP, it can
still happen, but over months if not a couple of years.
Oak stained black through ferrous reaction

Removing the black stain from Oak:

To get rid of the black stain from Oak (or other timbers) we use Oxcillic Acid.

Warning! Before you start using Oxcillic acid, read the safety
data sheets (SDS) that you can find on the manufacturer’s website (there should
be a web address on the container) and follow all safety precautions within the

  • Wear safety goggles
  • Wear a mask
  • Wear latex gloves
  • Use in a well-ventilated area

You will also need the following:

  • Clean metal container (to mix the solution)
  • Paintbrush (to apply the solution)
  • Scales (to weigh the crystals)
  • Wooden stick (to stir mixture)

The oxcillic acid is available usually in crystal form ready for mixing. It can
be mixed with water, which is what I have used below, or with Methalyted
Spirits; the advantage of using Meths is you get a slightly quicker drying time
for the mixture, which in turn means you can give the newly cleaned-up Oak a
coat of stain as protection quicker.

Mixing the Oxalic acid

Once you’ve got your safety gear on, we can move to mixing the Oxalic acid
solution. To mix the oxcillic acid, you need around 1 pint of water to around
60 grammes of Oxalic acid. Adding any more than the 60 grammes of acid will
not make the solution any stronger. To mix, I just used a piece of wood/stick.
As an aside, mix the solution in a metal container as it can melt plastic! Add
the water to your mixing container and then add in the 60 grammes of acid
crystals. Keep stirring until you have a few undisolved crystals in the
sediment of the solution.

Oxalic acid mixture

Thc mixed oxalic acid should look like this!


Once the mixture is ready, apply it to the stained Oak (or other timber) using a
paintbrush, being careful not to drip the solution on yourself or anywhere else.
The mixture is basically brushed on as if you were painting, no special
technique or anything needed! As the timber I am cleaning is not very badly
stained, it was cleaned up in less than an hour. Depending on how bad the
staining on your timber actually is, you may have to repeat this process a few
times: if so, then let the solution fully soak in and dry before adding more
acid solution and it will take considerably longer. The whole bleaching process
can last up to one to two hours, so just give the acid solution time to work
before adding more to the wood.

Black stained oak treated with Oxalic acid

The stained Oak five minutes after applying the Oxalic acid

The stained Oak twenty minutes after applying the Oxalic acid


When the solution has been applied, it is just a case of wait and see. When you
are satisfied that your staining has been removed, you just need to throughly
wash off any residue and leave it to dry before treating the doors or gates.

Black stained Oak treated with Oxalic acid - 50 minutes after treatment

The stained Oak fifty minutes after applying the Oxalic acid

Black stain removed from oak

No more black staining on the Oak, it just needs washing done and we’re good!


Remember to safely dispose of the container used for mixing the acid, along with
your stirring stick, gloves, paintbrush and any unused Oxalic acid.

To avoid the black staining in the future:

To avoid this happening again, then as I have said, all ironmongery should be
non-ferrous. Marine grade stainless steel is best, along with brass, however,
again, as I have said, the choices for gates and garage doors are limited so
your best options are as follows:

  • Marine grade stainless steel (grade 316 / A4)
  • Brass (but in truth, brass ironmongery for gates and garage doors is

Then your next best in order (though it will still ultimately corrode but you
should get a fair few years):

  • Premium black or ‘black on galv’ (this is galvanised then powder coated in
  • Hot spelter galvanised (this is steel that has been dipped into molten zinc,
    makes for a corrosive resistant coating

Avoid these common ironmongery finishes:

  • Black powder coated (this is just bare steel underneath and will soon
    corrode on Oak)
  • Bright zinc plated / BZP / Zinc plating (A very light form of galvanisation,
    a poor choice with Oak or similar timbers as it will not last long at all).
  • Self colour (this is untreated steel and only usually used internally and
    certainly not on Oak!)

43 thoughts on “My oak has gone black

    1. Indeed I did, a couple of typos that slipped through! Thank you for the heads up!

  1. Mike Edwards says:

    Did this and it worked brilliantly….I go a little less strong and multiple times and use a hard bristle brush to rub with the grain. My Green oak frame was manhandled into position using some metal jacks, Acroprops etc, so you need to be careful and have plenty of disposable pad pieces. My main ridge was circa 550kg and at 3.5m high it awkward to get to. Oxalic Acid was great… diolch yn fawr

    1. Jon C says:

      Glad it worked for you!

    2. Tracey says:

      Hi Jon, my oak beams have started to turn black so we have painted on oxalic acid crystals 3 times which seem to be working but we are not sure if it has a water proof coat (varnish) on them, should we carry on and applying this? Also would it help to put some sort of oil on after and what would you recommend?
      Many thanks

      1. Jon C says:

        The oxalic acid should take the varnish off as well or any other coating off. Regarding protection for future if they are somewhere they are likely to get wet/damp then this would help to stop them turning black again, although it could be some fixings (screws/nails/bolts) etc that are causing this.

  2. sibs says:


    do you have any ideas about how to remove coloured stain from exterior iron wood?
    the wood was stripped back and clear oil applied in order to keep the colour pale as possible.
    someone has applied a colour to a couple of the frames and they have turned very deep reddish colour so don’t blend with the other frames.

    is there a way to remove the stain or at least make it much lighter. the other frames look a very light oak colour.

    1. Jon C says:

      As far as I am aware oxalic acid will have little to no effect on the painted/stained surface. Best bet is to either sand it off or on a test piece try some paint stripper or similar and see how you get on!

  3. S Young says:

    I have an oak worktop treated with Liberon finishing oil. Someone left something on it (not sure what) and there is now a large stain. Sanded worktop down quite a bit and it looked quite good but stain came back as soon as re-applied Liberon. Now would like to try using Oxalic Acid but not sure if I have to sand worktop down again before using it, which would be a pain. If so, can I just sand down the stained area?

    Thank you.

    1. Jon C says:

      Hi Sy,

      To be honest I’m not 100% sure I’d want to be using Oxacilic Acid on something that I would be preparing food on, even if you seal the timber after.

      I’d suggest contacting Liberon before trying anything else, I hear they are pretty good at helping people out.

      Let me know how you get on!

      1. Alice says:

        Oxalic acid occurs naturally in plants we eat such as rhubarb and spinach. It’s what give you that funny feeling on your teeth. However, it should not be consumed in larger quantities. Personally (as a chemist), It’s not something I would be concerned about in this instance as you are unlikely to be taking much from the wood surface onto your food,

  4. Jane Smith says:

    Thanks v much for the information and advice on this post. We spent ages restoring an old garden bench and when the lovely new oak turned black we thought it was ruined but found this solution to work a treat – couldn’t be easier 🙂

    1. Jon C says:

      Great stuff, glad it worked!

      1. Sha says:

        I’ve got pine wood slatted fence panels. Had them installed last year in the summer when I did a major garden renovation project. It was untreated pine wood cut locally.
        I used Osmo oil UV protection stain. Ive been told by Osmo company that it’s a vegetable oil base that absorbs into the wood.

        Issue is that live in a area that gets very heavy rain and last winter I noticed my wood turning black. I thought it was a fungus or something and it looks like the tannin staining you’ve described. It’s made my beautiful wood grey/ black colour.

        The question I have is do I need to sand the wood down before applying the Oxillic acid solution? Or can I just apply it over the Osmo oil stained wood?

        Have many applications would you advise… Once doesn’t seem to be enough. Its very black.

        Many thanks


        1. Jon C says:

          If it is pine then it is unlikley to be tannin stain. It’s more likely water has got into the timber, then dried and what you are seeing is a black/blue fungal stain, whilst only comestic it’s highly likley that you won’t be able to remove the staining either by sanding. bleaching or oxillic acid.

          For the reason of rain we do not recommend using oil on any external timber, it’s made for the Scandinavian market really and although cold it does tend to be dryer. In the UK (if that is where you are based) it tends to wash off.

          1. Sha says:

            Thank you for your advice. Is there no way of removing this black fungus? There must be something on the market?

            I was thinking of using Myland dewaxing solution to remove the osmo oil first, then try using another solution to attack and remove the black mould such as Oxcillic acid or something else.

            I can’t be the only one with this problem. Yes I’m from UK and had heavy rain last winter. I live is a very leafy, green area.

            Your help and advice is sincerely appreciated ?

          2. Jon C says:

            If it is blue/black fungal stain then nothing will remove it. Assuming the stain doesn’t show on both sides of the timber then you can remove it by cutting away timber that is stained and planting on new timber in it’s place which seems overkill.

  5. Jason Brown says:

    Only just found this as over the weekend rubbed down my front doors with sugar soap and fine wire wool!!
    They were slightly sun damaged and flaky varnish, now they look terrible with black staining in the grain.
    will the above Oxalic acid treatment remedy the issue or are they going to need some other process?

    1. Jon C says:

      Give it a go on a small area first, it should work!

  6. Caroline says:

    Hi, I have lots of floor to ceiling oak panels in my new home and over the past 5 years the floors sills have larges water marks, can I use this ?

    many thanks

    1. Jon C says:

      Should be ok but make you sure you ventilate the rooms during and after using the acid

  7. Alan says:

    Hi i have read your piece and have followed the instructions to the letter to try and remove the black from my oak porch but after 3 coats applied on 3 separate days i dont think it looks any different to be honest is there anything else you could suggest that may help or should i just keep applying more coats .Many thanks for your time.
    Alan Smith

    1. Jon C says:

      It all depends on how deep the black staining has gone into the timber, if it’s only on the surface then the acid will help remove it. If it’s gone deeper than that then it won’t be much use.

      You could try a slightly stronger mix of the acid and see if that has any effect?

  8. Matt says:

    Hi Jon C

    Random one for you, i am restoring an old whisky barrel and the oak staves have gone dark, potentially due to metal hoops and also potentially due to what was in the barrel.

    Sanding doesnt seem to lighten it up much, do you think this acid would make a difference at all on this?


    1. Jon C says:

      Don’t see why it wouldn’t work, I’d avoid using the acid if you intend on using the barrel for more Whiskey however as doubt the oxalic would be much good for you!

  9. Alice Sowerby says:

    I love this guide!

    A couple of things to note – acid solutions should not be mixed, stored or used in a metal container as it will react with them (this is actually why it is good at removing iron stains) see

    Plastic containers are fine, as are glass.

    To those who were asking – oxalic acid will not remove paint or varnish, wood must be stripped first.

    1. Jon C says:

      Thanks Alice!

  10. Kat says:

    Hi Jon, I am working on a lake house built about 8 years ago that has gorgeous exposed Douglas Fir beams — many of the beams that are exposed on the exterior of the house have developed black streaks and spots. I’m not 100% sure, but I believe they could be water stains. Do you think oxalis acid would help remove the black streaks?
    Great article!

    1. Jon C says:

      It could well be ‘blue staining’ (despite it’s name it can be blue or blue-ish black) and is caused by the timber getting wet.

      If the staining is just on the surface then it can be sanded off (or removed using Oxalic acid), if your beams have take a soaking then it could go right through the timber and nothing other than painting them will hide it.

  11. Jean James says:

    Hi, i have untreated european oak sleepers ( i was told they were treated) there is black tar like substance weeping out, and it’s developing black marks that make it look dirty..firstly are they safe around pets and is it only the acid treatment i can use and will it work………gardener had already laid these before i knew they were untreated…

    1. Jon C says:

      A2 is better than nothing and any barrier between the timber and fixings should work! If using the acid then I’d keep pets away and clean up throughly before they are allowed anywhere near the treated timber.

  12. Paul says:

    Hi Jon. Many thanks for this informative article.

    I’m having trouble sourcing appropriately sized A4 stainless steel threaded inserts for an outdoor project. How much of a compromise would A2 stainless be? Also, if I used epoxy to seat A2 inserts, any idea whether this would this help to form a protective barrier?

    Many thanks!

  13. Martin B says:

    So i used oxalic acid on my barrel and it came up looking like new, treated it with a UV resistant oil, but now after only a couple of months its turning dark again and i fear in another couple of months it will be black again.
    Is this anything to do with it being used previously used for storing Bourbon, would there have been tanins in the bourbon ageing process thats causing this ?

    1. Jon C says:

      I’d say it was more likely the oil you used not doing it’s job or needing to be recoated. We don’t tend to recommend oils on our products as they need recoating every 2-3 months, so I’d say this is the reason.

      I’m no real expert on whether there are tanins within bourbon, other than what will have leaked from the oak to the bourbon during the aging!

  14. Jon says:

    Hi Jon. Great post. Very informative. Will the oxalic acid solution work for black marks caused by plaster staining on unfinished oak?

    1. Jon C says:

      It should work, yes!

  15. Beaula Page says:

    Hi there, will this work with black rainwater stains on Iroko gates please? It’s where the Osmo finish has worn and rain has got onto the wood, and where there’s been rain ingress in other areas, e.g. around spindles. I have seen other methods suggested to use bicarb paste, or white vinegar, or hydrogen peroxide.

    1. Jon C says:

      Hi Beaula,

      It may work with your water stains, however the staining can penetrate quite deep. If it is just surface staining then may help but the staining can penetrate quite deep in which case it may need sanding (and even sanding may not get rid of it)!

  16. Abbie says:

    Hi Jon

    We recently built a lovely oak porch in summer last year!
    But we didn’t stain it and it has recently got very wet over the winter and blackened/gone silvery quicker than we thought and we’d like to get it back to its more woody/orangey before we stain it.
    Can we use this method on our porch?
    I have a picture but can’t upload it on here

    1. Jon C says:

      It could well work all I can suggest if you give it a try somewhere it won’t be seen and see how it goes.

  17. Wayne Osborne says:

    Hi Jon. I’ve just install green oak sleepers and stupidly applied Osmo oil UV protection stain. I only installed a week ago and on top some are already going black.

    To be fair it hasn’t stopped raining since I installed them. Now I read you say that I will need to reapply every 2-3 months, what would you recommend I apply instead? I’m looking for that natural honey looking finish, which they looked like last week!

    I’m assuming all the rain has made them go black, and Oxalic acid. Do I need to let them dry out?

  18. Wayne Osborne says:

    Hi. Could you advise best protection for green oak sleepers?

    1. Jon C says:

      They’ve more than likely discoloured due to the water bringing the oils out of the timber, unless they are in contact with non-ferrous metalwork. If the staining is caused just by water then no amount of Oxalic acid will remove the staining as it will usually go into the timber quite deep.

      Green oak, you wouldn’t usually treat green oak with anything as it’s got quite a high moisture content with being ‘green timber’. I’d tend to just used a coloured wood presevrer to be honest.

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