Black stains in wood

[Crinan12]

The varnish on my hatch has cracked and peeled off over the winter. It's left all these black stain lines everywhere. Is there a way of getting them out before sanding and varnishing ?
Thanks

 

[geem]

The varnish on my hatch has cracked and peeled off over the winter. It's left all these black stain lines everywhere. Is there a way of getting them out before sanding and varnishing ?
Thanks

Oxalic acid will remove the black. We buy it in powder form and mix it with water. A tea spoon to a cup of water. Paint it on and leave it for a while and keep re-applying until the stain goes. If it is stubborn use a scotch pad to rub it in.

 

[jwilson]

As Geem says - oxalic acid. Get old varnish off first. Read https://www.highlandwoodworking.com/woodworking-tips-1111nov/alannoel.html

 

[Dan Tribe]

Agree, oxalic acid is the stuff to use. It's very toxic so wear gloves and a mask.
Make sure you neutralise it and wash it off thoroughly, it can delay drying of new varnish.

 

[jwilson]

Agree, oxalic acid is the stuff to use. It's very toxic so wear gloves and a mask.
Make sure you neutralise it and wash it off thoroughly, it can delay drying of new varnish.

Not saying not to take care, but "very toxic" is a bit OTT.

 

[FlyingGoose]

Not saying not to take care, but "very toxic" is a bit OTT.

In its pure form it will burn skin , and the lining of your airways and inside your lungs, why diluting it and being careful is always a good idea
But we as sensible boaters know this especially when rubbing down antifoul and applying it

 

[Dan Tribe]

Not saying not to take care, but "very toxic" is a bit OTT.

I was given a jar of oxalic acid crystals by a wood finishing guy. Before he put them into the jar he put on a mask and gloves and told me to do the same when using it and don't eat or drink until you have washed your hands. I've no idea of the concentration but I have always followed his advice. I assume that sanding the wood after using oxalic would also need caution.
Perhaps I'm being over cautious.

 

[Crinan12]

Thanks guys I've ordered some

 

[Crinan12]

Gave the oxalic acid a go. It's faded the stains a bit but they are still very much there. That was with 5 applications and letting it dry out in between. Perhaps the stains are too old or deeply engrained in the wood or something like that
Thanks

 

[upcountry2]

Oxalid acid works best in higher temperatures anything below 10c I find it doesn’t do much 20c it performs very much better. Having said that it will not get rid of every mark.
I have re veneered items such as you describe if finish can not be brought back to acceptable.
Small pieces of suitable veneer can be had from E bay for not a lot and stuck down with contact cement.
Better than original finish.
Hope this helps....

 

[Crinan12]

Thanks I didn't know that about the temperature thing
I'm doing it in the house thought so it's warm enough
Its definitely much better. I'm going to try sanding but I don't know how deep the stains go
I didn't think about the veneer stuff. I look into it
Thanks

 

[dunkelly]

i used to run furniture factories , it dosnt work on all hardwoods but if it is teak it will , but the secret was a saturated solution in hot water with repeated application they will come out .alternatively look up A/ bleach solutions from j penny antique services , this will bleach the wood which can then be re-stained if necessary .

 

[Crinan12]

Thanks
Maybe it had the solution a bit weak. It's definitely working just not as well as it does on the you tube videos !
Does sanding help? Can I sand out the stains?

 

[dunkelly]

without seeing the top first hand i cant be certain . i assume its solid wood and not veneered (do check)so sanding would eventually take the stains out but you may have to go quite deep . Try to persevere with the oxalic acid first , there isnt much i have found that a combination of both wont remove .

 

[Pcd]

Agree with all the above particularly temperature. On bad stains I have left the oxalic on for 24 hours + before neutralising and the time seems to have made a difference. For belt and braces and if I have the enthusiasm it's sodium percarbonate/oxygen bleach (several washes) followed by oxalic (several ) and then borax. Work each in with a softish brush. Usually happy with the results. Sanding always a last resort.