Stain under varnish

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Shanty Man
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Stain under varnish

Post by Shanty Man » Mon Mar 23, 2009 4:44 pm

On a recently bought boat I have dark stains under the varnish that I would like to get rid of before varnishing ( one or two pot I have not decided yet). The wood is sound and it is not the white "bloom" you get when damp has been drawn up to the varnish surface usually by the warmth of the sun. Almost certain it will have to be taken back to bare wood to do this although the varnish itself is not bad and would prime a coat of varnish if I ignored the dark stains.

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Re: Stain under varnish

Post by ACB » Mon Mar 23, 2009 6:03 pm

It's almost certainly damp.

Scrape the vanish back and leave the wood to air for a bit and it will clear up.

The more difficult problem is "where did it come from?" It may be either that the old varnish has cracked and become slightly porous, or it may be that the damp is coming through from the other side.

See the starboard bouyancy tank, the slightly darker patch, here (this prompted a rebuild...) :


That tank was no longer watertight and water had got into it after a capsize or three...
Last edited by ACB on Mon Mar 23, 2009 6:50 pm, edited 1 time in total.

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Re: Stain under varnish

Post by Nessa » Mon Mar 23, 2009 6:42 pm

On the MOGO there were black marks under the paint. I sanded these over lightly and the warm weather has done the rest. I think they just needed to 'breathe' to dry out. I'm fairly confident they came from whjere the hull had simply been lying on the grass. The paint over them just fell away.
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Re: Stain under varnish

Post by JohnK » Mon Mar 23, 2009 8:04 pm

On my old 12 I found that stripping the varnish with a hot air gun did both jobs at once. When I had to leave it for a year due to a back injury, the black stains were harder to get rid of, but careful applications of diluted Domestos (other brands of bleach based cleaner are available) did the trick.

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Re: Stain under varnish

Post by RichardB2 » Mon Mar 23, 2009 9:23 pm

It could well be stain. When I obtained Merlin 908 (1958) the decks had been stained and varnished, badly!! But what was worse it looked more like boot polish! I stripped back to bare wood and then used P40-60 oxide paper and a shed load of elbow grease to rub back to original wood colour. (Tip: gently brush off the sanding dust as it make ideal filleting power when mixed with epoxy) then apply single or two pack varnish. The big plus to using 2 pack on older boats, according to Mervyn Allen (big on vintage merlin restoration), is the resins absorb into any minor abnormalaties that come with older boats providing additional strength.

Good luck

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Re: Stain under varnish

Post by roger » Mon Mar 23, 2009 11:40 pm

Oxalic acid can be used to lighten wood that is stained. Search for Oxalic acid on this site and you will find loads of info.

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Graham T
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Re: Stain under varnish

Post by Graham T » Mon Mar 23, 2009 11:58 pm

I keep trying Oxalic as I hear from many places that it is the stuff to use but have never had much in the way of results with it. However if you need some there was a chap on e-bay that was selling it for a very good price - I was amazed to have it delivered the next morning when I had only ordered it at 4.30 the night before.
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Re: Stain under varnish

Post by Garry R » Tue Mar 24, 2009 9:41 am

Amazing that you can get it on ebay when it is a pretty potent poison and you can't get it from a chemist!!!

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Ancient Geek
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Re: Stain under varnish

Post by Ancient Geek » Tue Mar 24, 2009 9:56 am

This will be brief (Blackberry!) The answer is a lemon! Cut a lemon in half and rub one half on the stain cut the remaining half into quarters put one quarter into a large glass add ice Tanquerays gin about 1/3 of the way up fill up with Fever Tree Tonic sit back and enjoy when you have finished the drink the stain will have gone wipe down with thinners turps or whatever you use finish in the usual way, may as well use the other quarter of lemon too.

Shanty Man
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Re: Stain under varnish

Post by Shanty Man » Tue Mar 24, 2009 5:59 pm

Oxalic acid is good for rust too. But because of its on the poisons register I have used lemon juice including Jif and white vinegar. So it depends if you prefer pancakes to chips. But thanks for your help.

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