- Because brown spots occur after an overall cleaning, the entire carpet will not need to be cleaned again. Mix a mild acid cleaner or acid rinse, available from most janitorial supply houses. Though "acid" sounds scary, it only means you will be cleaning with a chemical that has a pH reading of less than 7.0. Prochem Chemicals, based in Chandler, Arizona, manufactures Yellow Rx specifically for removing brown-out spots. Yellow Rx has a pH of about 2.0. Keep in mind that lemon juice and vinegar are examples of safe acids. However, always follow the manufacturer's safety recommendations.
As with any cleaner, test in an inconspicuous area first. Then, apply with a squirt bottle by lightly misting the affected area. Keep in mind that the spot was likely caused by overwetting the carpet in the first place. Therefore, use the least amount of chemical as possible.
Agitate the area with a brush or sponge. Then thoroughly blot the area with a clean, dry white towel. Repeat the procedure until the brown spot is no longer visible. - Once the spot is clean by hand, it is a good idea to remove the excess moisture with a carpet extractor or wet vacuum. Rinsing with clear water may be necessary to neutralize certain acid cleaners (consult the manufacturer's instructions). Adding water to the spot is fine, just be sure to pull out as much as possible with the machine.
The key to avoiding the return of the brown stain is to leave the carpet as dry as possible after cleaning and rinsing. Therefore, use several dry passes with the extractor. - To avoid browning of the carpet, whether from overwetting or wicking, rapid drying of the carpet is recommended. Place a professional grade air mover at the trouble spot to dry the carpet. If one is not available, then use a household fan.