Darker blotches in sanded wood?

[DanielWestern]

Hi - I recently purchased a second hand piece of furniture and have been sanding the wooden top down to remove the surface gunk and imperfections. However in a couple of areas I notice some dark blotches in the wood that don't seem to be going away with more sanding (see pics). Any idea what this could be? How to get rid? Would something like wood bleach or oxalic acid help here? Intending to finish it with Danish oil when its ready so would need to find a solution that would be compatible with that. Thanks!

 

[phillc]

The lower picture looks like water marks - the bits of rings left over from someone putting a mug or plate down?

 

[Cubist]

As @phillc suggests a couple of them look like water stains but they could also be oils. Try a little methylated spirit on a cloth first. If they are oil then the meths should start to lift the oils out and, as for water stains, some additional sanding may then remove them in full. There is a technique used to remove stubborn oil stains but that's a little risky as it means pouring a little meths onto the stain and setting it alight but a heat gun and a soft cloth pad can do the same job. Some of those darker stains appear to be bruises of the wood caused by glancing blows from hard objects. These can be difficult to remove because they can be much deeper than a surface stain and you could end up sanding divots out of the wood. A trick used by some restorers that sometimes works is to apply the edge of a hot clothes iron through a soft cloth. This has the effect of slightly raising the surface of the wood to remove the dent and allow easier and uniform sanding but that may not remove the dark stain and for that you may need to use a bleaching process.

By the way, that's oak, but what is the piece you are working on?

 

[DanielWestern]

Thanks a lot both for the replies @Cubist and @phillc .

Its the top part of a TV unit that we picked up in a charity shop, so I was not 100% sure of what kind of wood it is, but suspected solid oak.

One thing I forgot to mention which might be helpful is that the wood in the dark marks look slightly different to the wood around them, almost "shinier", and when looking at them from above do not look as visible, but when looking at a more oblique angle such as from the front look a little darker, due to the way they reflect light. Does that help at all? Is it just a different quality of that part of the wood where the marks are that is causing this? Or does it still sound like it could be related to oil stains? Would bleaching be advised in this case?

 

[LadyArowana]

The small mark right at the bottom of the last picture looks a little bit like a burn, careless cigarette left on the edge? It’s a nice solid piece of furniture and a few marks doesn’t detract too much, and probably won’t notice so much once you have completely finished.

 

[DanielWestern]

Thanks - oh yeah that one at the bottom of the pic I am not worried about, it was more the weird blotches elsewhere tbh.

Yeah its not the end of the world either way but it would be nice to get it looking at good as possible if we are investing time in "upcycling" it (hate that word), so if there is a way to get rid of the blotches then great. If not then its no big deal.

 

[Cubist]

Thanks a lot both for the replies @Cubist and @phillc .

Its the top part of a TV unit that we picked up in a charity shop, so I was not 100% sure of what kind of wood it is, but suspected solid oak.

One thing I forgot to mention which might be helpful is that the wood in the dark marks look slightly different to the wood around them, almost "shinier", and when looking at them from above do not look as visible, but when looking at a more oblique angle such as from the front look a little darker, due to the way they reflect light. Does that help at all? Is it just a different quality of that part of the wood where the marks are that is causing this? Or does it still sound like it could be related to oil stains? Would bleaching be advised in this case?

Give the shape and direction of travel implied by those marks I would be thinking of transfer from say a rubber or plastic foot but it's possible that they are natural colour features of the wood grain. I would try some wire wool dipped in methylated spirits to clean them up and that would also help to remove any residual hard finishes that are still sitting in the grain after sanding. Bleaching you have to be careful about theres a definite art in the timing of how strong you make the diluted bleach and how long you leave it on the piece before applying the neutraliser - you must not leave it overlong.

 

[jamest]

When I looked at the pictures in small format and read your comment about them being 'shiny' and it being oak, it occurred to me that some of them might be medullary rays. Looking at them in larger format makes me doubt it, but if they aren't anything else, perhaps some of them are.

 

[Kearn]

I’d say all those in white circles are knots or natural areas of the grain. Enjoy them for what they are. Oak can be very interesting, with a ll the medulla rays in the grain when the right section of tree is cut the right way. I’ve got some lovely ‘cats paws’ burrs here and there in my frame…

The rest simply look like dents, where sanding hasn’t reached. Do these bits by hand with your finger to get the undulation.

A bit late now, but I’d not try to take it back too naked.

 

[DanielWestern]

Thanks @Kearn - yeah I should have been a lot clearer in my original post, apologies... the marks I am talking about are the ones in the ovals in the pictures here:

They aren't really dents, they are flush with the surrounding wood, and like I said up there seem to have a slight sheen to them so they look darker when viewed at certain angles (which makes me think they are not just oil stains or other kind of stain). Very odd.
Like I said its not the end of the world, but even if I can't do anything about it I would like to know what they are, even just to satisfy my own curiosity!

 

[Kearn]

I still would go with some kind of impact trauma. Might feel flush, might even be flush, but the localised timber may be compressed in those spots meaning their absorption of whatever is more concentrated. I don’t know, They just look mechanically applied … the way they drag across the grain.

 

[Cubist]

I heard a tale years ago of a woodworker who found some hard and difficult to remove black marks in a new piece of timber. While working to remove these he came across some fragments of lead. Someone had grazed the tree while bumping off squirrels or birds with his trusty shotgun.