How to Remove Black Stains From Oak

Oak is renowned for how hard and sturdy it is, making it an iconic choice for high-quality furniture and building material. High levels of tannic acid contribute largely to oak's durability, but those levels are also responsible for most black stains on oak surfaces. Iron materials, like hinges and nails, interact with tannic acids to create those large black stains. Other sources of staining would be mildew or simply a darker commercial stain or multiple layers of lighter stain. There is a different correction to each of these causes.

The first rule when staining wood is to always start with a lighter color than you want or expect to have at the end of the process. Wood is naturally porous and locks in chemical stains surprisingly fast, so starting light and adding more to darken the color as desired is much easier and more straightforward than starting too dark and having to try to correct it afterward. Dark wood stain colors can be difficult to change once applied.

The primary method of lightening a dark commercial stain on the wood is to sand it down to remove the layer affected by the stain. After that, you need to start over the entire process. This method is not only time-consuming but costly. The woodworking concept of measuring twice and cutting once holds as true for staining wood as it does for shaping it.

Mildew is a kind of mold that is inherently difficult to remove altogether. It comes from extended exposure to moisture. A lack of direct light or airflow will accelerate mildew and mold growth, but moisture is critical. The traditional approach for removing mold and mildew is applying a one-to-one solution of bleach and water, but some research has shown that this merely stunts the mold rather than killing it.

Alternatively, you can try using distilled white vinegar and hydrogen peroxide, though to improve hydrogen peroxide's stain-removing properties, it's best combined with baking soda. The baking soda combination enhances the oxidizing properties, breaking cell walls and penetrating surfaces to lift out stains. Note that if it is left in place too long, hydrogen peroxide can bleach surfaces lighter than you might have wanted.

The go-to solution to this kind of deep-set iron oxide staining is oxalic acid. This compound naturally dissolves the iron and tannic component, resulting in a complete lift of the black stain from the wood within an hour of application. Some household items contain oxalic acid, like potatoes. However, the pure chemical is preferable to the potato option. It doesn't have extra components, like starches or iodine, that could complicate the chemical reaction or create undesired complications even after application.

It is extremely important to use rebreathing equipment, good ventilation and protective garments, such as gloves, to avoid skin contact when using oxalic acid. Remember to follow all manufacturer instructions when using concentrated chemicals or cleaning products and pay special attention to keeping oxalic acid crystals moistened to avoid potential poisoning.