Michael Dresdner

straight talk about wood finishing

Unwanted stains

Q: I just bent some cherry sides for an acoustic guitar. A reaction must have occurred during the bending process resulting in green blue spots on the wood surface. What do you recommend to eliminate or minimize?
A: Removing stains depends first on know what caused them. Blue green stain is most likely one of three sources: coming in contact with natural or artificial stain, such as extractives from a different wood bent in that press; a reaction with a metal surface; or blue green mold. Sadly, each has a different fix.
Contact with a natural or synthetic dye will usually come out with a wash of laundry bleach. That will not change the color of cherry substantially. Contact with a natural pigment is usually only on the surface, and will sand out fairly quickly. A reaction with a metal can usually be reversed with a wash of a ten percent solution of oxalic acid, and that won’t harm the color of cherry either. If it is blue green mold, about the only thing I know of that removes it is a wash with a fairly concentrated (30% or higher) peroxide solution, and that will substantially darken cherry.
Do some detective work first to figure out the likely source and the fix should be easier.