By hand or having to use a machine does depend on how bad the yellowing is, I reckon. The worst car on the UK roads for headlight yellowing is the Renault Clio. I did one last week where I had to go down to a pretty abrasive grade of paper to eliminate as much yellowing and pitting as safely possible (still had a bit left but same as polishing paintwork, have to balance end result against integrity of what is left). This is the before: Then used a DA Sander with various grades, followed by a MO