Caustic Stripping Fiasco

      A cautionary tale: Doors sent to be dip-stripped came back blackened. Finishers troubleshoot and suggest fixes. July 11, 2005

I am looking for some help regarding some doors that I sent to a paint stripper who dipped them in caustic soda. When I saw them, I was appalled. They seemed to have turned to a dark brown/black color. I only wanted the varnish removing to reveal the natural grain of the wood which was Tornillo. The doors and frames that I have manually stripped using Nitromors look light and natural.

How can I best get rid of the dark stained effect caused by the caustic soda treatment? Can I have the doors sanded or bleached to make them light and natural again?

Forum Responses
(Finishing Forum)
From contributor M:
Can you explain more about what you said that "The doors and frames that I have manually stripped using Nitromors look light and natural". If that was the case, I'm wondering why you brought the doors in?

From contributor D:
Caustic stripping should be followed by an oxalic wash and clear water rinse. This does two things:

1. This gets rid of the darkening which is caused by the alkalines reacting with tannins in the woods.

2. This neutralizes the alkalines which may still be present in the woods.

You need to rinse the oxalic acid with a wash of water because as the acid dries, the water in it evaporates leaving you with the acid crystals in their original powder form. This powder is an
extreme irritant which will create breathing problems and will burn/sting your eyes. While the liquid acid (the powder dissolved into water) is often applied with a bug sprayer, a pressure washer is used to rinse the residue off.

From the original questioner:
To Contributor M: I stripped several doors and some window frames manually with Nitromors. They had been previously stained and varnished with Sikkens. Then I noticed a firm advertising to strip doors, and as my workload was increasing I thought I would send six doors to them. My problem now is to restore the doors treated by caustic stripping so they will match the other doors and frames stripped manually. I intended to wax the doors using a brushing wax.

From contributor R:
It sounds like a your doors were dumped into a vat full of lye, and after an hour or so they were takin out and the slush was hosed off with a high pressure water hose. At this point, Oxalic acid won’t do much good as the wood has been chemically burnt by the lye solution. Have you tried a cabinet scraper to remove the top layer of wood? Sanding is another way to get rid of the burn, but it, along with the scraping, will also eliminate any sort of patina. You could try bleaching out the burnt surface, it might work, but I would try any remedy on the back of the doors first.

From contributor M:
I would suggest going with a stripper that has a flow-over system next time. You will need to sand the woods both from the lye and the water rinse. If you need to lighten the wood further, you can use household bleach. You may need a few applications and be sure you wear the proper personal protective gear.

From the original questioner:
I tried oxalic acid with little effect. It looks as if the original varnish has been baked into the wood. The acid and wash left the black formation untouched. I then reverted to using Nitromors and a cabinet scraping tool. Although it's harder to get off than the original varnish, it is scraping off and signs of the natural wood are appearing.

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