Dark stains in wood - getting rid of?

[Tintin]

I am sure I have seen threads on this, but can't find them with the search engine so.....

What methods are there for getting rid / reducing dark staining of wood interiors (usually caused by damp).

I have had two suggestions so far.

1. Sand down then bleach or use polycell mould killer then re varnish

2. Sand down, heat up, scrape with a blade, re varnish.

Thoughts on these methods? or other methods?

Thanks.

 

[Tranona]

You need to remove all the existing varnish to see what the underlying wood is like. Sometimes just removing the damp is enough. However the usual treatment is to use oxalic acid on the discoloured bits.. You may find it difficult to get an even colour, particularly if it is a veneer and you don't want to sand too heavily or you might go through the veneer. Can be useful to use a light woodstain such as Coloron to get a more even colour and then a satin or matt varnish which is more tolerate of colour variations than high gloss.

Can be a lot of work and probably best tackling one panel at a time. Hardwood trim is usually easier to deal with as you have more wood to work with when getting back to a consistent colouring.

 

[robertj]

hot air gun (carefully) with very sharp scraper then oxalic acid painted or sponged on.
more coats of oxalic acid until the black disappears.
use either bleach then or Teak Wonder rejuvenator which will lighten the teak to original colour
careful of sanding as any panels will be veneered so will quickly be ruined.
then what ever top coats you prefer.

 

[robertj]

tranona's reply appeared while I was engrossed in typing lol

 

[AntarcticPilot]

You need to remove all the existing varnish to see what the underlying wood is like. Sometimes just removing the damp is enough. However the usual treatment is to use oxalic acid on the discoloured bits.. You may find it difficult to get an even colour, particularly if it is a veneer and you don't want to sand too heavily or you might go through the veneer. Can be useful to use a light woodstain such as Coloron to get a more even colour and then a satin or matt varnish which is more tolerate of colour variations than high gloss.

Can be a lot of work and probably best tackling one panel at a time. Hardwood trim is usually easier to deal with as you have more wood to work with when getting back to a consistent colouring.

All good stuff - the only thing I'd add (being halfway through a project to re-finish my interior woodwork) is that you have to remove ALL the varnish before using Oxalic Acid; otherwise you get uneven colouring where old varnish has stopped the Oxalic Acid reaching the wood below.

Concerning removing varnish, I've found careful sanding and/or scraping works OK. But as others have said, be careful - the veneer isn't all that thick. I am going to try a hot air gun next time I work on it.

 

[PlankWalker]

Cheap hot air gun works a treat removing varnish, expensive ones can get too hot.

Plank

 

[nigelmercier]

I'd add (being halfway through a project to re-finish my interior woodwork) is that you have to remove ALL the varnish before using Oxalic Acid; otherwise you get uneven colouring where old varnish has stopped the Oxalic Acid reaching the wood below...

I'm not so sure, I've let it soak into the areas where the varnish was damaged, it seems OK.

But what do I do now? Sponge rinse it? How do I know when it has dried out?

 

[AntarcticPilot]

I'm not so sure, I've let it soak into the areas where the varnish was damaged, it seems OK.

But what do I do now? Sponge rinse it? How do I know when it has dried out?

You've been lucky, and perhaps the wood underneath your varnish hasn't darkened (as opposed to staining) as mine has. - I failed to remove all the varnish from a floorboard, used oxalic acid, and it was very patchy; dark where the varnish remained and light where it had been removed. That said, the varnish was in good condition, so it really was protecting the wood underneath it. Fortunately, I was able to remove the remaining varnish and another application of oxalic acid gave a uniform colour.

I just wash it off with a wet sponge; that seems to be fine. And I then leave it until it looks dry; the colour change is easy to see.

 

[Blueboatman]

Scrape, bleach, as said..But you can bleach more than once..One trick is to scrape and bleach locally, then 'test' for colour balance by wiping with white spirit.. the white spirit resembles closely the finished varnished effect..then bleach again if still too dark. When its v close, stop bleaching and just build up patch layers of varnish before giving the whole panel a couple of full coats.

If you remove all the varnish across the whole panel at once, how are you going to prevent overbleaching of undamaged adjacent areas?
Of course, if the whole panel has been exposed to sunlight for a few years it may be generally faded ( and often the fade is a gradient across the boat/panel, hmmm)

I personally do not strip every panel to solve one or two darkened areas. The risk of damage, digging in, scratches, ripples, unevenness, sheer time involved far exceeds the perceived improvement- ( imho/ime of course ).

And if its really knackered in one patch, think about letting in a new piece of veneer-its not that difficult and can be fun and rewarding. Straight strip or diamond, there are plenty of good craft books on the subject..Winter projects eh?

EDIT. Aha, AntarcticPilot is saying pretty much the same thing.

 

[nigelmercier]

I was a bit concerned about leaving acid residue, so I've now gone over the whole area with sodium bicarbonate solution, then rinsed. Then soaked with methylated spirit.

 

[Blueboatman]

Meths is fine stuff but it is water based and will 'raise the grain' requiring a light sanding afterwards..