How to use Oxalic acid, and what about oxygenated water?

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?

TIA Holly

Can you get it at that sort of concentration, though? I thought it was pretty nasty stuff in high concentrations.

We used to use oxalic acid to whiten-up aluminium panels, (it whitens by removing copper, afair).

Pure chemical oxalic acid does leave a white crystalline deposit, that is difficult to shift if left to dry out.

We had better results and far fewer cleaning problems with our d-i-y oxalic acid..... ie, rhubarb juice.

'Nasty' is a relative term. It'll cause some nasty burns, but no worse than concentrated HCl. You can get it at 90% v/v from chemical suppliers.

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.

Good luck

Another Dave

yep, didn't mean nasty in a hysterical it'll-escape-and-kill-all-your-family way, but you certainly wouldn't want to go splashing it around.

ok, didnt know that.

so, where did I put those plans for that rocket-powered dragster......

Thanks.

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.

Thanks again Holly

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

It's likely to be due to the tannin in oak, a search on this should turn something up:

cheers, Pete.

Many of the chemical suppliers are quite happy to supply to individuals, as long as the individual is prepared to provide a name and address.

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.

Oh I don't know, I'd positively encourage neighbour's children and pets to have a lick.

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.

Andy.

Andy.

Actually so would I - particularly the pets - but you can't say things like that in places like this.

Cheers

Another Dave

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.

Holly

it'll-escape-and-kill-all-your-family

dragster......

Much better idea, and better still for being d-i-y but alas we don't have any rhubarb. Might try to find a friend who does though, thanks, Holly

Thanks for that.

Wasn't really thinking of french polishing the joists :-)

Ahem, I'm afraid I am old enough (just!) to remember a threepenny bit :-)

Holly

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.

Thanks again Holly