Blue/black stains on koa from bending sides

If you have a string instrument of any kind that needs fixing, a mistake you made in building a new instrument that you need to "disappear," or a question about the ethics of altering an older instrument, ask here. Please note that it will be much easier for us to help you decide on the best repair method if you post some pictures of the problem.
Post Reply
Marc Boyd
Posts: 2
Joined: Tue Feb 28, 2012 1:52 pm

Blue/black stains on koa from bending sides

Post by Marc Boyd »

I've recently bent some koa sides & now have the notorious ugly blue/black stains (I'm assuming they're from the spring steel slats on my bending machine). I tried to prevent this by wrapping the sides in aluminum foil, but some water obviously leaked through and caused the staining. I would like to avoid too much sanding as the sides are .085" - not to mention the stains will obviously not just be on the surface. I had also read that a stain remover such as CLR (used to remove calcium and hard water stains from kitchen applicances) might work, but I didn't have any luck with it. So, does anybody have anything up their sleeve to deal with these stains? (other than an opaque lacquer)?? :x
Much appreciated!
Marc

User avatar
Charlie Schultz
Site Admin
Posts: 1342
Joined: Fri Dec 23, 2011 6:53 pm
Location: Atlanta, GA

Re: Blue/black stains on koa from bending sides

Post by Charlie Schultz »

Hi Marc and welcome! Someone should be along soon with some info for you. Certainly if you have some leftover koa, you might want to create the same kind of staining on it and run the tests (e.g. CLR) on that before you attach the finished sides.

John Hamlett
Posts: 110
Joined: Thu Feb 02, 2012 10:59 am

Re: Blue/black stains on koa from bending sides

Post by John Hamlett »

Hydrogen peroxide may help, if the straining isn't too bad, but you may loose some natural color too in the process. Charlie is obviously correct that you should test on scrap.

User avatar
Mark Swanson
Posts: 1972
Joined: Thu Jan 05, 2012 11:11 am
Location: Grand Rapids, Michigan USA
Contact:

Re: Blue/black stains on koa from bending sides

Post by Mark Swanson »

Oxalic acid works. I have used a product that I got at an auto parts store called "Rust Out", made by Casite. It comes in a spray bottle and has oxalic acid as a main ingredient. You spray it on, and as it evaporates the stain just disappears!
I looked on the Casite website, it looks like they don't make it anymore. I looked around, there are a few spray-on products that would probably do the same thing. I found one called "Evapo-rust", another called "Rust-Off", and I see Harbor Freight sells both the Evapo-Rust and another called "White Ox".
  • Mark Swanson, guitarist, MIMForum Staff

Marc Boyd
Posts: 2
Joined: Tue Feb 28, 2012 1:52 pm

Re: Blue/black stains on koa from bending sides

Post by Marc Boyd »

Thanks for your help everybody. I found a local supplier for Evapo-Rust and will try it on scrap. Will also keep my eyes open for anything else with oxalic acid.
Mark S: we stayed on the same floor at the last GAL - was great to meet you. Hope you're doing well! It was only when I got back home that I realized that an old MIMF article I had saved on your Spruce/Paduak Fatboy was your handiwork. Nice one! See ya in 3 years...

User avatar
Mark Swanson
Posts: 1972
Joined: Thu Jan 05, 2012 11:11 am
Location: Grand Rapids, Michigan USA
Contact:

Re: Blue/black stains on koa from bending sides

Post by Mark Swanson »

OK, I remember meeting you now! Good luck, see you next time!
The "White Ox" stuff sold by Harbor Freight looks good, comes in a spray bottle and is probably perfect because it says it can be used on wood.
  • Mark Swanson, guitarist, MIMForum Staff

User avatar
Greg Robinson
Posts: 686
Joined: Fri Jan 06, 2012 1:54 pm
Location: Coburg North, Victoria, Australia

Re: Blue/black stains on koa from bending sides

Post by Greg Robinson »

I have not used Koa myself, but I have heard that it reacts with aluminium foil, causing this type of staining. The solution was to wrap it in butchers paper or something similar.
Can anyone confirm this?
MIMForum staff member - Melbourne, Australia

User avatar
Charlie Schultz
Site Admin
Posts: 1342
Joined: Fri Dec 23, 2011 6:53 pm
Location: Atlanta, GA

Re: Blue/black stains on koa from bending sides

Post by Charlie Schultz »

Not aluminum foil, but my last guitar was koa and I bent the sides with a StewMac heating pipe (which is aluminum)- I did not notice any staining.

John Hamlett
Posts: 110
Joined: Thu Feb 02, 2012 10:59 am

Re: Blue/black stains on koa from bending sides

Post by John Hamlett »

Iron + moisture + tannins = blue/black. Koa (among other woods) contains a pretty high concentration of tannin, so it needs to be kept away from iron and moisture at the same time. That is almost surely the source of the staining. For this one, a remedy is needed, next time prevention is the key.

User avatar
Greg Robinson
Posts: 686
Joined: Fri Jan 06, 2012 1:54 pm
Location: Coburg North, Victoria, Australia

Re: Blue/black stains on koa from bending sides

Post by Greg Robinson »

Thanks Charlie. I'm sure that there was a wood that reacts with aluminium, guess it isn't Koa. Anyone know? Or am I crazy?
MIMForum staff member - Melbourne, Australia

Jay Gordon
Posts: 4
Joined: Mon Mar 05, 2012 1:15 pm

Re: Blue/black stains on koa from bending sides

Post by Jay Gordon »

you can also get a small quantity of oxalic acid at most paint stores. just add a bit to a cup of water, wipe on the koa, brush with stiff brush, rinse with clear, clean water. Always works for me.

Michael Lewis
Posts: 1472
Joined: Thu Jan 12, 2012 1:22 am
Location: Northern California USA
Contact:

Re: Blue/black stains on koa from bending sides

Post by Michael Lewis »

A long time ago there was a fairly involved discussion on the old forum regarding blue/gray stain from bending with metal, which may be archived. It even occurs with stainless and can USUALLY be sanded off but not always. I would try sanding a bit before applying any chemicals that can change the coloration of the wood.

Post Reply