Bleaching wooden hull

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NorfolkNick
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Bleaching wooden hull

Post by NorfolkNick » Fri Oct 30, 2015 2:45 am

Hello,

I have a clinker Broads Half-Decker that I have just scraped the varnish off, its from 1946 and made of pitch pine on oak ribs.

I would prefer to re-varnish rather than paint it, but the boat has been neglected for a few years by the previous owner so there are quite a few dark areas where the varnish has lifted and damp has got in.

I have been reading online about "bleaching" such areas, either with normal bleach or with oxalic acid, and was wondering if any members had any experience of doing this, I guess my questions are as follows...

How should I prepare the surface prior to bleaching.
What substance should I use.
Should I use full strength or dilute.
How long to leave it on for.
How to effectively neutralise the surface once sufficiently bleached.
Anything else I should know or look out for?


Thanks
Nick
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Michael4
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Re: Bleaching wooden hull

Post by Michael4 » Fri Oct 30, 2015 11:53 am

Search oxalic acid on this site and you'll get an awful lot of information and opinions. There has been much discussion in the past.
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Ed
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Re: Bleaching wooden hull

Post by Ed » Fri Oct 30, 2015 2:18 pm

.....Oh, but talking about Oxalic acid is such fun.... :-)


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chris
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Re: Bleaching wooden hull

Post by chris » Sat Oct 31, 2015 11:01 am

If its stain caused by water Oxalic should be successful.
If you have scraped the varnish I would go over it with some paint stripper now just get any remain stuff out of the grain otherwise the result could be patchy.

Mikey
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Re: Bleaching wooden hull

Post by Mikey » Mon Nov 02, 2015 11:52 am

Like Chris, I have used Oxalic acid many times with great success. Remember to wash treated area, when bleached, with water to remove residual acid.

Michael Brigg
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Re: Bleaching wooden hull

Post by Michael Brigg » Mon Nov 02, 2015 2:20 pm

I have often wondered "why Oxalic Acid?"

Why not a solution of Napisan (Sodium Hypochlorite) as a weak bleach producing simple salt and CO2 as it's end reaction so little if any need to endlessly wash out the product.

There is alot of interesting information here...

http://www.antiquerestorers.com/Article ... bleach.htm

It seems that Oxalic acid is particularly favoured in bleaching out "Black wood" and clearly this is a bit of a traditional dogma, but things have changed a bit since the dogma developed.

Oxalic acid is now a legaly classified poison, and so it is hard to obtain from the chemist. When I bleached my firefly @ 40years ago, you simply went to the chemist and purcest a 500g jar of the stuff. These days you probably need to by a pre mixed (and exeassivly dilute version of it, or in a proprietary cleaner such as "Stain devils rust remover, where 10gm costs @ £3 -£5. Good for rust spots on sails but far to little for a hull.

It appears that Oxalic acid came to be used alot for wood because of the prevalence of Iron staining. Iron bolts allowed to rust or perhaps iron staples, later removed, but leaving small residues. Oxalic acid is particularly good at removing rust. (Not sure why, I'll have to revise my Chemistry notes for that. But I am happy to accept that on trust.)

Apparently even iron traces on tap water is sufficeint to react with the wood Tannin and produce nasty watermarking stains.

I suspect back in the day when wire wool was often used as a fine grade abrasive , lots of potential rust spots would be left in the prepared surface, and this lead to widespread use of Oxalic acid and scubbing with water, in the routine preparation od wood hulls before revarnishing to prevent the finish being ruined.

If however used on wood to restore the fresh wooden coulor, the effect I think can be disappointing. Yes it will remove the Iron stains, though in my experience the "black" wood becomes grey. Sometimes enough to then accept a little wood stain to match its surround... but elswhere it is really no mor effective than any other bleach. The advantage of Oxalic Acid is that it is a WEAK acid, so the bleaching effect is likely to be less, with less (or no) Deadening of the surrounding new wood coulor.

The brightening effect on the hull is produced as much as anything by a good scrub (nessessary to remove the Oxalic acid dried crystals) or scraping of the oxidised 1/2mm or so of the surface to get a "new wood" effect. (Herein lies the value of a really sharp Oil stone sharpened blade pulled along the line of the grain, being sure that you do not end up pulling against the grain.

Scrubbing out the Oxalic acid will of course also (if using tap water) reintroduce traces of Iron.

Most importantly do a test area. The main benefit is to be gaind in woods such as Oak, which has alot of Tannin.
Michael Brigg

chris
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Re: Bleaching wooden hull

Post by chris » Mon Nov 02, 2015 3:27 pm

Oxalic, in powder form, is still on Fiddes' cataloque
http://www.fiddes.co.uk/product.php?nam ... cidsbleach,

Interesting, Michael. certainly there are other bleaches/cleaners. It is surprising how bare timber on a hull can be improved if you simpley wash well with a cleaner such as flash. Oxalic shouldn't bleach the colour out of timber though whereas some bleaches will. Chlorine as used in swimmingpools also has its uses. but it is a question of matching the right stuff for whatever the stain is.

I've often wondered how a dacron main sail can get rust stains when there seems to be no contact with any ferrous metal. So it in the water!

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Re: Bleaching wooden hull

Post by Nigel » Mon Nov 02, 2015 3:55 pm

oxalic acid is freely available on eBay.....Got mine there a while ago

Nigel

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Ed
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Re: Bleaching wooden hull

Post by Ed » Mon Nov 02, 2015 5:11 pm

But I think that Mike got the main points to my mind:

First, If you are not careful, the wood goes grey and looks horrible.

Second, it is amazing how much you can improve wood, especially the brightness and colour, simply by scrubbing with a stiff brush and vim or whatever.

Generally, I don't like bleaching and prefer to accept the black marks. But like the clean non-black areas to look good, hence the advice to just scrub.

I have had some luck with Netrol for general cleaning, but really seems to work best for darker woods and since they changed the recipe, both Neil and I have had some real issues with the stuff and I would now only use it for dark manogamy, but not ply, especially gabboon and lighter woods.

Lastly if you are doing it at this time of year, try and make sure you can get it nice and dry within a reasonable time. I had some horrible mould marks turn up a while ago after cleaning the decks of my Jollyboat, that were horrible. Would of preferred black stains to orrible grey marks.

cheers

eib
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Max McCarthy
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Re: Bleaching wooden hull

Post by Max McCarthy » Mon Nov 09, 2015 6:42 pm

Michael Brigg wrote:I have often wondered "why Oxalic acid?"

It appears that Oxalic acid came to be used alot for wood because of the prevalence of Iron staining. Iron bolts allowed to rust or perhaps iron staples, later removed, but leaving small residues. Oxalic acid is particularly good at removing rust. (Not sure why, I'll have to revise my Chemistry notes for that. But I am happy to accept that on trust.)
Michael, to save searching your chemistry notes - Oxalic acid is a reducing agent, hence is able to reduce iron(III) oxide (i.e. rust) to pure iron, and remove the stain:

H2C2O4 + Fe2O3 -> 3H2O + 6CO2 + 2Fe

I believe this is the right equation...

Cheers,

Max
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Ed
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Re: Bleaching wooden hull

Post by Ed » Mon Nov 09, 2015 9:18 pm

FOFL.....

that is a bit like teaching eggs to suck grandma...

he he

eib
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Rupert
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Re: Bleaching wooden hull

Post by Rupert » Mon Nov 09, 2015 10:09 pm

Teaching eggs to suck grandma... the mind boggles.

Max's post was an education. Thank you!
Rupert

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RonnyDG
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Re: Bleaching wooden hull

Post by RonnyDG » Mon Nov 09, 2015 11:43 pm

"Reduces Iron-oxide into pure Iron"... That explains why the stains in sailcloth I nicely remove with Oxalic acid always just re-appear after a while; There is iron residue left in the fibre that turns brown again.
Greetings,
Ronny

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Max McCarthy
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Re: Bleaching wooden hull

Post by Max McCarthy » Sat Nov 14, 2015 9:46 pm

Ed wrote:FOFL.....

that is a bit like teaching eggs to suck grandma...

he he

eib
Hmmmm, no sorry, you've got me there, I'm familiar with the phrase in reverse order, but you may have to elaborate on this one?
Rupert wrote:Teaching eggs to suck grandma... the mind boggles.

Max's post was an education. Thank you!
You're welcome Rupert....!
RonnyDG wrote:"Reduces Iron-oxide into pure Iron"... That explains why the stains in sailcloth I nicely remove with Oxalic acid always just re-appear after a while; There is iron residue left in the fibre that turns brown again.
Precisely....

Cheers,

Max
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