Rust stains that you discover on wood during a refinishing project are certain to slow you down. Rust is notoriously difficult to remove. This difficulty is all the greater with wood, for the surface of wood can easily be damaged by energetic efforts at rust removal. Stains from old nails and metal furniture seem to become part of the wood itself. They are a result of a reaction between water and the tannic acid inside the wood. Rust stains go deeper than abrasives can reach. But it is possible to remove rust stains when refinishing wood. The process requires preparation and some powerful chemicals, so proceed with care.
Clean the wood. The rust-stained wood must be stripped down to the surface. Dirt, wax, and finishes will interfere with the removal of the rust stains.
Get some oxalic acid, or wood bleach. Oxalic acid is an organic acid that occurs in some foods. Concentrated, it also serves as a powerfully caustic cleaner. It is used in many preparations for refinishing wood, and is often marketed as wood bleach. You’ll find the wood bleach powder necessary to follow these steps in a hardware or home remodeling store.
Prepare the solution. The oxalic acid powder must be mixed with hot water in order to be used on the wood. Follow manufacturer’s instructions, and adjust the proportions for the size of your rust removal job. Prepare the solution in a plastic or ceramic container, as metal reacts with the acid.
Apply the solution. If the rust stain is a small one, use a thin brush to apply the oxalic acid solution directly to it. Allow the solution to dry thoroughly. Depending on the size of the treated area, this may take an hour to as long as overnight. If the rust remains, apply the solution a second and a third time, always letting it dry, until you’ve achieved the desired result. Oxalic acid lightens the color of the wood it touches, so the former site of the stain may be brighter than the wood surrounding it. To reduce this spotty effect, use a cloth to apply a light coat of the solution over the entire area. An area of wood broadly affected by rust stains can be treated with the solution using a larger stiff-bristled paintbrush.
Neutralize the acid. Stop the corrosive action of the acid with a neutralizer. Mix 2 tbs. of baking soda with 1 qt. of water and use this alkaline solution to wash the wood. After this dries, wipe the surface with a dark cloth. If you find no powder residue, your wood is clean. If powder residue remains, clean, neutralize and test again.
Refinish the wood. The wood treated by oxalic acid will be rough in texture and must be sanded. After the wood is smooth, proceed with your refinishing project.
Items you will need
- Oxalic acid
- Hot water
- Mixing stick
- Small, narrow paintbrush
- Cleaning cloths
- Non-metal mixing container
- This solution has to remain hot in order to work. If it should cool during the process of refinishing wood, heat it again without letting it come to a boil. Do this by placing the container within a pan of hot water.
- When you use oxalic acid, it is essential to protect yourself. Wear gloves and goggles. Avoid splashing the solution. Oxalic acid can burn. If it touches your skin, wash thoroughly. If it gets in your eyes, see a doctor.