Legal Question in Personal Injury in California

It would protect you psychologically, but not from liability for intentional acts or if you knew of a dangerous condition of the property and did not tell him.

No waiver is iron clad and although you're not liable for conduct on independent contractors, you're liable for their work. A person does not see inherent risks when hiring a contractor that they will be harmed. Also it is against public policy to waive all risks that are not apparent.

Liability for what? Your biggest risk is that by hiring an unlicensed contractor you become their employer and the employer of every painter he hires. That makes you strictly liable for workplace injuries, and no waiver of that is allowed. Your only protection would be to buy workers compensation insurance like any other employer. I can tell you, though, workers comp for painters is expensive due to the ladders. In fact I believe only roofers and maybe electricians pay higher rates in the construction trades.