I got this beautiful old keyboard from my friend, it is an Apple mechanical keyboard from 1986. Why do I love this keyboard? because it’s mechanical, and I believe that a mechanical keyboard has a longer age than a membrane keyboard. The keycaps mostly use PBT plastic, but the plastic cover case and the space button are made with ABS plastic so it is yellowing over time. So I decided to restore the plastic using an H2O2 solution.
Before restoration
This is the picture before the restoration 
PCB
This is the PCB that made by Apple Computer 1986.
Inside the cover
Three colors yellowing
Use the 12% H2O2 solution
After result
I got no photos when the keyboard is under the sun, so here is the result after a couple of hours under the sun.
Maybe it can be more white, but the current weather is cloudy and not very good for retrobrighting, maybe I need a UV light to do indoor retrobrighting.