Seems a little extreme to me - what is the surface finish on the joists? Are they treated at all?
What colour are the stains? Are they just water marks?
Both the oxalic acid and the peroxide treatements rely on the strong oxidising power of these chemicals. They will efectively act as a bleach, to remove coloured stains. But they may also discolour the wood in the process (lighten it).
I have a problem with water stains on oak joists, caused by spouse leaving velux windows open for ventilation in the rain! People here seem to recommend oxalic acid to remove the stains and after much hunting around shops in France I have finally found a product which contains it. Our local sawmill man also recommended "eau oxygenée" 90something per cent, available from the chemist apparently. Would that be hydrogen peroxide??
The instructions on the oxalic acid product say that it must be hosed down or power washed with copious amounts of water after the application of the oxalic acid. Given that our joists now have white painted plasterboard above them and a tiled floor underneath, a power washer doesn't seem a very good option! Does anyone know whether either the oxalic acid or the eau oxygenée can be used carefully in small quantities dabbed on or whatever and carefully rinsed off with sponge and bucket, or is all this water really necessary?
Oxalic acid is very poisonous if ingested. It is also an extremely unpleasant way to die. If you must use it be very careful not to allow children, pets etc. near.
Green oak cut a couple of years ago, dried in a shed. Not treated but sandblasted to give them an 'old' appearance to match existing timber and to remove chainsaw marks on some of them (don't laugh, this is diy remember!)
Yes, just water marks, but they vary in appearance. Some are not so dark and over a largish area, some are darker where the water doesn't seem to have spread so far so all the stain is concentrated at the edges.
If I decide to get either or these products at all, depending on responses here, I can test it first on a spare bit of oak. The marks are on about five out of about 30 joists so if it were a case of having to bleach all of them to match I probably wouldn't do it at all. I will probably treat all of them eventually with the tinted preservative that I used on stairs and other woodwork to even up the colour of all the oak in the building. This in itself would disguise the stains somewhat I think.
I have an old book, Titled " French Polishing and wood surface finishing". It mentions the use of Oxalic acid to lighten dark stains. I quote- " When a piece of wood has been stained too deeply, it can be made lighter by using a bleaching mixture. As much Oxalic acid as can be placed on a three-penny piece, dissolved in a cupful of hot water, will make up a solution of suitable strength, but do not use a cup for the purpose, as oxalic acid is very poisonous. Paint this on to the wood that is too dark, and when the proper colour has been approximately reached, sponge over with vinegar. IMPORTANT-- On no account omit the final sponging with vinegar. Failure to use it will cause the french polish to crack within a year. " Unfortunately I cannot find a date on the book ( I would guess 1930ish) , so I will let you decide what size of threepenny bit it was :-) Hope this helps. Cheers Tom
Probably metal tannates. Oxalic acid will shift them, hydrogen peroxide won't. Most "wood bleach" for deliberately lightening wood is peroxide - a two bottle preparation applied one after the other.
There are two easy sources for oxalic acid. One is "Barkeeper's Friend", a white powder you can find in hardware shops or supermakets. The other is "patio deck cleaner", generally a mixture of detergent and oxalic acid. Either will work for de-staining oak. Apply the powder on a damp cloth.
I have used oxalic acid: it was easily available as 'wood bleacher' at a local DIY place. I used it to lighten some pine that had been darkened by rot and exposure. It was reasonably successful in this regard. I applied several coats and then sponged it off several times with plain water. I was slightly worried because the smell of the oxalic acid was still in evidence after several spongings. In the end I decided that there was no way I could get rid of every last molecule of it and simply let the area dry naturally before proceeding to apply preservative then advanced woodstain. No ill effects are evident viz the woodstain 3 months on, but we'll have to see! No instruction regarding dousing with vinegar was given on the carton.
We are in France, where nasties generally seem to be more readily available. The sawmill man told me that the chemist would sell it to me if I said that the sawmill man had sent me and what I wanted it for, also that it would be less expensive than the oxalic acid products. That said, from replies here I think the oxalic acid will be more suitable so will have a go with that.
Not important this, but where would the metals have come from, would have thought there would be only traces of metals in the green oak??
I'm going to take your word for that since you always seem to give good advice here, and stick with the oxalic acid. Then again, the sawmill man usually knows what he is talking about and he understood the exact nature of the problem when he recommended the H2O2.
I forgot yet again to mention that we are in France. I have seen you recommend Barkeepers Friend before. It took me ages to find anything containing oxalic acid in hardware shops and builders merchants. Maybe that was why the H2O2 was recommended :-)
Didn't think of that, might have found something like that here more easily if I had looked in the right place.
The product I found was in solution, quite possibly along with a detergent as you suggest, no mention of the concentration, just that the active ingredient was oxalic acid. I'll get some of that and experiment a bit going carefully with the product and then sponging with water or might try the vinegar. The main thing is that from what people are saying it should be possible to do it without the copious amounts of water - if that had been necessary there wouldn't have been much point in buying the stuff.
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