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 Post subject: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Wed 22, 2020 10:48 pm 
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Location: Woodinville, WA USA
A friend sent me a KLH Model Twenty One radio for repair. Its cabinet had a big black stain. No idea where the stain came from; it could be the classic wet flowerpot mildew damage, or perhaps spilled ink or some other blackening agent. I have read about using oxalic acid to bleach out mildew stains -- sometimes with very impressive results -- so I got some to try. Here's the cabinet before bleaching (but after I stripped the cabinet using a 50/50 blend of acetone and lacquer thinner):

Image

I happen to own the same model radio in fine original condition, so I took a "before" photo showing the two cabinets side by side:

Image

The bleach package instructions are pretty useless ("Remove bleach after obtaining a satisfactory result"), so I started cautiously, applying the bleach solution and then flushing it away after 5 minutes. There was a modest, but noticeable lightening of the black stain. On the next round, I left the bleach on the cabinet for 10 minutes. Similar result -- modest lightening, but no miracles. Next time around, 20 minutes of bleaching. I let the rinsed cabinet dry completely between bleaching rounds. That was enough bleaching for yesterday.

When I resumed this morning, I mixed a new batch of bleach, doubling the concentration that I used before, and leaving the bleach on the cabinet for 30 minutes, dabbing the wet areas occasionally to prevent them drying out. After two rounds of this, the stain is definitely lighter, but still quite visible. Here's another side-by-side photo:

Image

The stain is actually somewhat lighter than it appears in that photo. In any case, this is not the miracle cure I was hoping for. This lightened stain will be less obvious when I restore the original cabinet color with toning lacquer, but it will still be noticeable.

Is this typical of mildew stains? Or am I possibly looking at a stain (from ink -- who knows what?) that will never be completely removed by oxalic acid?

Trying to sand through the stain is out of the question. I could sand all the way through this very thin veneer in less time than it took to type this post.

Yes, I could simply replace the top veneer and finish it to match the other veneered parts. But as long as I've got this bleach thing going on, at least I can try to learn a little more about the process :)

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 12:05 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Indianapolis, IN
It works best when it's hot -- mix first, then microwave it for 20-30 seconds (but be careful -- hot acid!)

Before:

Image

After:

Image

The remaining mark is actually a burn, not a stain.


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 12:37 am 
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Location: Woodinville, WA USA
Sure, I can try heating it for the next application. I have been mixing it up with hot water as directed, but I didn't try reheating the juice for every application.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 1:34 am 
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Joined: Sep Wed 03, 2014 11:06 pm
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Location: Poulsbo, Wa
I've used oxalic acid to remove black stains on wood with some luck. It is my understanding though that the stain has to be
water related in some form.

Since you may not know for sure it is worth a a try.

Bill


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 4:12 am 
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Location: Baguio City, Philippines
If the stain isn't from being wet, you may get better results with Chlorine bleach. Regular household bleach can be tried, although a stronger concentration such as swimming pool bleach (dry calcium hypochlorite you mix with water) would work better.


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 5:47 am 
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Location: Woodinville, WA USA
Well, heating the solution in a microwave didn't make it work any better. I'm afraid I have reached the point of diminishing returns with this particular bleach. The last couple of applications made a marginal change, if any.

Whatever this stain might be, oxalic acid isn't going to erase it. And I'm beating up the wood with so many long immersions in a water-based solution.

I'll fill the grain, apply toning lacquer, and see how it looks. If it's just too awful, then off with the old veneer and on with the new.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Thu 23, 2020 3:58 pm 
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Joined: Sep Wed 03, 2014 11:06 pm
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Location: Poulsbo, Wa
When you heat the water it allows more of the oxalic crystals to dissolve ie a stronger solution.
When i do this i wait like 60 seconds. If it hasn't lightened by then it wont.

Bill


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Fri 24, 2020 6:53 am 
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Just for the heck of it, I also tried some Clorox household bleach today, and it's just as useless as the oxalic acid. On to grain filling!

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Fri 24, 2020 4:55 pm 
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Location: Sunnyvale CA
philsoldradios wrote:
Just for the heck of it, I also tried some Clorox household bleach today, and it's just as useless as the oxalic acid. On to grain filling!

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html



It's not useless - it's just not a miracle. The original stain is by far the worst and deepest I have seen. I wouldn't even bother with the toning, it's not going to be OK. Just replace the veneer and move on.

Brett


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: Apr Fri 24, 2020 6:10 pm 
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Location: Pewaukee, WI
Brett_Buck wrote:
philsoldradios wrote:
Just for the heck of it, I also tried some Clorox household bleach today, and it's just as useless as the oxalic acid. On to grain filling!

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html



It's not useless - it's just not a miracle. The original stain is by far the worst and deepest I have seen. I wouldn't even bother with the toning, it's not going to be OK. Just replace the veneer and move on.

Brett

I have to agree.
If it hasn't significantly lightened since the last picture the cabinet would have to be toned almost black to hide the stain.
It's a flat surface so applying fresh veneer over the top should be straight forward.


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: May Sat 02, 2020 12:44 pm 
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hi this looks like an iron oxide stain to me I would try a little CLR (phosphoric acid) on the spot.
Rich


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 Post subject: Re: Using Oxalic Acid Wood Bleach on Black Stain
PostPosted: May Mon 11, 2020 2:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 934
Location: saginaw, mi USA
I had a cabinet that had a similar stain. I agree with Rich that it might be a rust (Iron Oxide) stain. I used a product found in the cleaning aisle called Whink.
It is a rust remover and completely removed the black iron oxide from my cabinet. Did not harm the wood and took stain.

Keith


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