Cover the floors in the work area with painter's plastic.
Put on protective gloves. Dissolve one half cup of trisodium phosphate in 2 gallons of hot water. Prepare one bucket of hot water for rinsing.
Soak a clean rag in the water containing the cleaner. Remove excess water from the rag by wringing it out.
Wash the walls with the rag soaked in TSP. Start from the top of the wall and work down. Rinse the walls using a clean rag and clean water. Use separate buckets for wash water and rinse water. Refill the rinse bucket with clean water when it becomes discolored.
Repeat washing the walls and then rinsing them until they are clean. Allow the surfaces to dry completely before painting.