Mixing borax with water will enable you to apply the acid to mold just as you would apply any liquid spray. Take note, however, that borax requires hot water for proper activation. Do not boil the water, but allow it to become hot before you pour it over your boric acid crystals. Cold or lukewarm water will not allow you to achieve all of the benefits of the chemicals.
For Large Surfaces
If applying boric acid to a large surface like a fence or deck, mix 1/2 cup of borax with 1 gallon of hot water. Spray the borax solution over the entire moldy area and do not rinse. Borax will not harm most household surfaces. You can allow it to remain on the surface and continue to proactively prevent new mold growth. Use a hair dryer to dry the area.
If you have black mold growth on your upholstery, mix 1/2 cup of borax and 2 cups of hot water. Use a sponge to absorb the borax and then gently wipe the moldy upholstery. This will ensure that you do not over-saturate the upholstery, a problem that can make mold growth worse. After applying the borax to the moldy area, use a hair dryer to dry.
When Not to Use Borax
Some surfaces are sensitive to even the weakest of acids, and therefore you should avoid using borax. These surfaces include limestone, granite, onyx and marble. These natural stone surfaces are porous, and as a result, they can trap moisture and develop mold. Since black mold grows best on cellulose (plant-based) materials. You will rarely find black mold on stone, but if you do, clean it using a solution safe for natural stone, like a stone cleaner or pH cleaner.