Splotches on wood.

[ShatnersBassoon]

When I finish my current build I’ve been contemplating on a huge slab of English Sycamore I have lying around which excitingly, may just be thick enough for a one piece guitar (36 mm). That is, if I manage to find somewhere that can mill this piece perfectly, because it’s not entirely even/straight along its length, it’s out in a few spots by a minute margin....I suppose I could just mill it all with the router.

Anyway, thoughts on all of that welcome, however my main point with this post is to see if anyone knows what is going on with these dirty grey splotches on the bottom side of the wood, do they normally plain out easily? What are they? I believe it is fully seasoned, however is it showing some sort of moisture damage here? 

Cheers.

Edited February 12, 2020 by ShatnersBassoon

[Bizman62]

That looks like some moisture related miscolouring - condensation combined with dust containing some metals maybe. You may have heard about ebonizing wood with vinegar and steel wool, something similar may have happened there. The only way to find out how deep it goes is to plane it. It doesn't look too bad, a hair off the surface should tell if you should worry or not.

Measure some of the smaller spots, plane the surface off and measure again. If the spots are significantly smaller and less colourful by only taking one or two shavings away, there should be no issues.

[ADFinlayson]

I'm not sure what the discolouration is, but I've got an ash body I'm working on and all of the boring uniform figure is perfect but all the interesting grain has that grey splotchy stuff, it's far more obvious under finish too, so I've had to cut it so using the boring stuff.

With regards to flattening, something I've done a few times is to knock the high spots off with a hand plane, then stick several rows from a roll of 80 grit to a thick sheet of mdf and rub the blank over it until it's falt - it doesn't take as long as you'd think. That also works well after using a router sled because router will never leave you a perfect surface.

[Bizman62]

The more I look at that the more I think about ebonizing! As you know, it's made by diluting steel wool (=iron) into white vinegar (=acid). You may also have heard of acid rain which obviously means there's acid fog and mist as well, and dust can easily contain iron particles especially near a workshop or a road.

Another option that popped up is ammonia fumes. Similarly to vinegar+iron the effect is stronger with woods containing more tannin. Maple seems to get that greyish look when fumed so that may be the reason for the splotches. 

If the reason is ammonia fumes, the source can be anything from a lawn fertilizer to plastic softener residues from a tarpaulin to household cleaning solutions.

Some good reading about colouring wood with chemicals: https://issuu.com/ksorsky/docs/art_of_coloring_wood

[ShatnersBassoon]

Thanks so much for all of the replies! I will let you know how I get on. I was thinking that I could spray it a cool colour, although that always seems a bit of a pity.