Stain on soundboard

[Bruce McC]

A black stain has developed sometime in the last 4-5 months on the soundboard of this guitar which I built in 2018. The soundboard is Englemann spruce and I sealed it with wash coats of dewaxed blonde shellac whilst working on it i.e. installing the rosette, then finished with Mini Wax Wipe on Poly Satin. The Mini Wax was applied with a cloth and rubbed back between coats with 0000 steel wool. There was no evidence of any staining or discolouration on the soundboard before I started building or when the guitar was completed in December 2018.
The guitar has not been exposed to extremes of heat or humidity. When not in use it is stored inside my home in a gig bag. The guitar was last played on a regular basis in September/October 2019 and there was no staining on the soundboard.
About a week ago when I pulled this guitar out of the gig bag I noticed this black staining on the soundboard. The stain is on both halves but most prominent on the treble side. I can not see any evidence of the staining on the underside of the soundboard.
The Mini Wax which I used was given to me. The tin was about half full and the contents were of a thin consistency it was not gluggy or viscous. I do not know how old the Mini Wax was.
I wonder if there has been some chemical reaction between the Mini Wax, the shellac and gassing off from material in the gig bag?
Comments and theories on why this staining has occurred would be welcome. Perhaps I could leave as is and say the soundboard is ancient spruce dug up from an Alaskan tundra.

[Mike Thomas]

Some reaction to contamination from the steel wool?

[johnparchem]

It is hard to tell but an interaction with something in the gig bag. I wonder as the somewhat localized stains.

[GregL]

Hi Bruce,

You said ".. The stain is on both halves but most prominent on the treble side ...", can you please post a pic showing the entire soundboard? I don't know what has caused the stains, but a pic of the entire soundboard will show if the stains are somewhat symmetrical or not, which might assist with diagnosis.

Thanks,
GregL.

[kiwigeo]

Occasionally I've had top sets turn up in my stash with a few spots of mould....generally they looked like this.

DSC_0129.thumb.JPG.2220f5e05de75adcf7cb5d52a047b05e.JPG (57.89 KiB) Viewed 18156 times

My only encounter with badly mould stained wood was a load of Lutz I received many years back from a Canadian supplier who is now dead. The sets he sent as a replacement were even worse.

Bad Wood.JPG (86.27 KiB) Viewed 18156 times

[Bruce McC]

Thanks, all comments appreciated.

Mike - yes I have considered contamination from the steel wool. I only used the steel wool lightly between the coats of Mini Wax and would like to think that I carefully wiped the surface clean before applying the next coat. That is my usual practice, I do not use steel wool on bare timber. As the stain is blackish in colour similar to iron staining it may well be from steel wool contamination.

John - I have several gig bags of the same brand and guitars stored in these bags have not developed any stains. However they are French polished not finished with Mini Wax.

Greg L - this is the best I can do with my IPhone. The staining on the bass side is feint and does not show in the photo. The pattern of the staining is not symmetrical.

Martin - I am certain that there was no evidence of mould on the bare/raw stock and if there was that I would have noticed it whilst building the guitar.

[kiwigeo]

Ok Bruce had intended not to mention where he got this top from but I'm going to declare myself as the supplier of the top so I can post up this pic which was taken shortly before I sent the top off to Bruce. This Englemann originally came from Graham Hein..and was part of a box of tops cut from the same billet. I've included one set from either side of Bruce's set for comparison purposes.

If the blue staining is to do with the use of steel wool then I dont now where the moisture will have come from. The Miniwax wipe on poly is is oil based isn't it?

[Trevor Gore]

Bruce,

I suspect that kind of staining came from the gig bag. Some time back a manufacturer was offering a "protective wrap" for a guitar for inside the case and it left marks like that, but fortunately it wasn't on one of my guitars. Whatever it was came out of the wrap and into the finish, though I can't recall what the finish or the "protective wrap" materials were. I didn't have to deal with the problem, so I don't know what the remedy is or even whether there is one. If I had to try to remove those stains, I'd start with a gentle wipe with Shellite on a soft rag. If that didn't work, there are not many places to go, as any more aggressive solvent will likely damage the finish. You'd need to get a very good understanding of the chemistry of the finish and the solvent to go any further.

[Bruce McC]

Martin - thanks for posting your photos, they confirm that there was no evidence of staining on my set ES 014 prior to use.

Trevor - thanks, I tried using Shellite on a small area of staining and it had no apparent affect.

So I decided to bite the bullet and I have sanded back a small section of the soundboard and it would appear that this stain
can be sanded out! I am happy to refinsh rather than retop After sanding the top back I will leave the guitar for a few days to ensure that the stain has been removed.

[kiwigeo]

Great to hear that stain sanded out Bruce.....I'd still love to know what actually caused the stain.

[Allen]

Doesn't look like you had to be too agressive. Has it gone into the spruce, or is it more just in the finish?

[Bruce McC]

Allen - yes it rubbed out easily by hand, it appeared to be in the finish and not in the timber.