WHAT IS THE BLUISH BLACK STAIN ON MY FINISH?
Probably iron stain, caused by a reaction between iron (usually from ungalvanized fasteners) and wood extractives. Black staining associated with iron degrades wood; it is generally not a problem with nails but can be significant with large bolts. If removal of such stain is desired the stain can be scrubbed with a dilute solution of oxalic acid. Handle the cleanser carefully as it is toxic, and rinse thoroughly with water.
