Mix in a bucket 1/2 cup of liquid soap and 1/2 gallon of warm water. Wear latex gloves to keep your hands clean. Stir the mixture with a sponge or soft scrub brush. Fill another bucket with 1/2 gallon of warm water and set the bucket off to the side.
Move any furniture, rugs or accessories sitting next to or on the FRP wall panels to another room or area.
Wring out excess mixture and scrub one FRP wall panel section with the mixture-soaked sponge or scrub brush. Re-dip the sponge or scrub brush in the mixture, wring out the excess and continue scrubbing the wall panel section until it is clean. For stubborn stains, spray the stain with a general-purpose household cleaner and let it stand for five to 10 minutes. Scrub the stain vigorously with the damp sponge or scrub brush.
Dip a large, soft cloth or rag in the bucket of warm water you set off to the side in Step 1. Wring out the excess and wipe the FRP wall panel you just cleaned to remove any mixture residue and remaining debris.
Re-dip the sponge or scrub brush in the bucket containing soap mixture and repeat Steps 3 and 4 for the next FRP wall panel section that needs cleaning.
Dry the wall panels with a clean, large cloth or rag.