In a momentary lapse of sanity, I offered to paint the inside of my mom's place in the next month. The painting itself I don't mind. It's the prep work and cleanup that are a major pain. Anyhow, her walls haven't been painted in about ten years and I know they are probably dirty and dusty. Even though I'm going to prime the walls, I know they need to be cleaned. What's the best way to do this? Should I just use dry cloths or do I need to use soap and water? Or do I need to use something else?