When your couch is exposed to warmth and moisture for a prolonged period of time mildew can grow and spread on the couch's material. Whether your couch has been stored in a damp, warm location or you're cleaning mildew that developed on your furniture as a result of water damage in the home, you'll want to remove the mildew from your couch as soon as possible to prevent the mildew from spreading further and weakening the material fibers on your couch.
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Vacuum off the entire surface of your couch, including cushions and underneath cushions, along with the back and sides of the couch. Vacuum thoroughly to remove mildew from the surface fibers of your couch. Discard the vacuum bag afterward so mildew doesn't have the chance to transfer elsewhere.
Combine 3 cups ammonia and 3 cups cool water in a bucket. Dip a sponge in the diluted ammonia solution then wring out the excess ammonia solution.
Blot the mildew stains on your couch with the ammonia solution. Press the sponge into the surface of the couch material to get the ammonia solution into the fibers to kill the mildew. Rinse your sponge frequently then continue to clean the couch with more diluted ammonia.
Wick up ammonia solution from the couch with dry cloths. Apply pressure to the cloths with your hands to soak up the moisture.
Wet a clean cloth with plain cool water to rinse the ammonia solution out of your couch. Dab at the couch material to rinse, then follow by blotting dry with clean cloths.
Remove as much moisture as possible from your couch using a wet vacuum with upholstery attachment. Move the upholstery attachment back and forth over your couch until no more moisture is extracted.
Direct a fan at your couch and allow the cool air to speed the drying process. Move the fan every hour so it's directed at a different part of the couch until the couch is dry.