Oak tannin stains?

[scholar]

Hello All

We have quite a lot of oak in the way of stairs, internal doors and a large built in cupboard. This is all “new” ie modern oak installed in the past 15 years or so. Some of the oak needs refinishing, but I would like some advice on some staining that has appeared over the years.

Some relevant points:
- the oak is a mixture of different jobs, ie bought-in solid oak doors, separately manufactured solid oak stairs and doors, and the large cupboard is a mix of solid oak and veneered panels/shelves.

- the staining has appeared seemingly randomly on all these different components.

- the building was left unused for long periods after the main work was done and so will have been relatively damp as the place was fully drying out (it is a conversion of a very old building)

- the finish should have been universally Osmo Polyx Oil, but I cannot be sure what was actually applied.

I have not yet attempted any heavy scrubbing/chemicals/solvents as I want to avoid making it all very patchy without knowing what I am doing!

I have included some photos below.

The first question is - are these tannin stains? If not, do we know what they are?

Second question is - is there a practicable solution to removing the staining?

Any experience/insight would be much appreciated

Cheers

 

[pe2dave]

Possible insight.
6 years ago I made and installed a tree seat. All American white oak.
With time and bird **** it is approaching the 'white' you show above.
Blame? Dunno. sunlight bleaching perhaps? Just time?

 

[scholar]

....the 'white' you show above...

Thanks - there may be some lightening in places due to sunlight, but that is not the problem (and my photos are maybe not very good).

Our problem is the dark brown spots that have appeared.

Cheers

 

[mrpercysnodgrass]

It looks as though you have some tannin stains and some mould. I think the first thing to do would be to wipe it over with some domestic bleach. Make sure you apply the bleach over the entire section, finishing with the grain, that way you will avoid any blotches. Apply the bleach neat using a kitchen sponge scourer (use just the sponge side) using the sponge means you can work up to but not over an adjoining section. Then just allow the bleach to dry. This should get rid of any mould and some other stains but will not get rid of tannin stains. So the dark grey/ blackish stains you have are probably a reaction to contact with iron (I’m guessing wire wool) the only way of getting rid of these is to either sand through them or to use a solution of oxalic acid. You can buy oxalic in two forms, crystals which you dissolve in hot water or you can buy it in solution, the only one I know of is ‘wood bleacher’ made by Liberon, I think it has meths added to it! Anyway it works very well. Apply this in the same way as the bleach but after it has dried and hopefully erased the stains you will need to neutralise it with water. If this all works you will then need to build up the finish, if it has had osmo already applied I would stick with that, put as many coats on as are needed to seal and fill the grain, that should stop any further staining.

 

[yetloh]

Oxalic Acid crystals are widely available. AG Woodcare are vg in my experience Oxalic Acid Crystals. It may not work on all your staining but quite a bit of it looks like iron staining and oxalic is definitely the way to go for that.

Jim

 

[scholar]

Thanks for the inputs chaps.

One further observation that had not previously occurred to me is that the stains are mostly apparent on horizontal surfaces.

That made the suggestion of mould quite plausible - I have had a small experiment with bleach on one area but it didn’t seem to shift much.

I am less sure it is staining from steel wool residue - in my experience this is more black and the stains here are brown. (Also, some of the wood I fitted and finished and did not use any steel wool).

I will get some oxalic acid (AG Woodcare is an excellent place near to us) and give that a try.

Cheers

 

[AJB Temple]

If it has all been coated with Osmo Polyx as you suggest (or similar finish) then I very much doubt that oxalic acid will do a lot. I build in oak quite a bit and oxalic in my experience has some effect on bare wood but is less of a magic cure for stains than people think.

I would select an area, and using a scraper remove the finish. If the scraper deals with it, then it is most likely mould. If it is iron staining then the scraper will improve it but not remove it usually. Once scraped, then try the oxalic acid. (I buy out in 5kg drums in powder form and mix as required).

Good luck.