Q: The feet of metal chairs have made black stains on my hardwood floors. Oil soap, powder cleaners and bleach do not have any effect on the spots. Ammonia seems to lighten them, but not much. Steel wool removes the wood finish before it has any effect on the stains. Are there any other chemical solutions I can try before I resort to sanding? - Tom Davis, Fort Washington, PA
A: Household chlorine bleach is ineffective on black wood stains, which most often are caused by moisture. Ammonia can darken wood (fumed oak is darkened with ammonia), as can the iron oxide in steel wool. So what works? Many furniture restorers prefer wood bleach for removing black stains. You can find two types of bleach: oxalic acid and two-part A/B wood bleach. Try the oxalic acid first. You should be able to find it at any hardware store. Rainbow is one common brand.
Oxalic acid is cheaper than two-part A/B wood bleach and requires just one application. It’s also the preferred chemical for removing black stains caused by metal reactions (such as the stains sometimes seen around nail heads).
Two-part A/B wood bleach (available at woodworking specialty stores) is often used to lighten the overall tone of wood, but it can be used to spot-treat stains. You apply the alkali solution and then the peroxide solution immediately afterward.
Whether you use oxalic acid or A/B bleach, you’ll need to sand off the finish topcoat over the stain area so the solution can penetrate. Read the usage information on the container before you begin. After the solution has either worked or dried, rinse the area with a neutralizing agent such as two tablespoons of baking soda dissolved in one cup of distilled water. - HANDY