Here are the images I have at present.
Tim's Vic-20 prior to Retr0brite. This had been stored by a previous owner inverted, on a desk in the sun with stuff piled on it. It looks dirty, but this is actually yellowing of the plastic.
I delivered the HP21 to Tim a couple of hours after he had started his Vic-20 case "cooking". His case is partially done here.
He tool a close-up of the HP21 to show the foam on it. We have a bloody great hole in our ozone layer here, so this probably works better/faster than for northern hemisphere people.
Tim only gave them 3 or 4 hours rather than the all day that is recommended. He used a 6% H2O2.
Here are 2 HP21s. They both looked pretty much the same colour previously. The one on the left is now almost completely un-yellow.
I might try a similar trick on the keys of this HP21. They're not that yellowed, but it would be valuable to know if the keys will be affected. This HP21 has a number of serious cosmetic problems already, so it won't be a huge loss for me if it goes pear shaped.