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 SDS.

  • 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 non-existent)

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 black)
  • 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!)

Comments

  1. Don’t you mean oxalic acid?
    https://en.wikipedia.org/wiki/Oxalic_acid

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