I heard and saw a video about doing the retr0bright with just sunlight few months ago : https://www.youtube.com/watch?v=8P1OVj0IcqY
I was curious to test it, no matter it being hoax or not. Risks were low. However, I had to wait To get good weather here in Finland. When the weather got better, I chose the piece to be tested with: Apple //e 65c02 (Nordic, swe/fin) unit’s lid was good candidate. It had yellowed but in the middle where the monitor had stand for years, was not much so. Therefore, there would be great contrast In addition, should get good results.
Therefore, I removed the logos from the lid, cleaned it and placed in the sun. After first day, maybe 5 hours of direct sunlight blasted the lid, but I could not see any difference. So I kept going, on and off when I had the time, for days, actually few weeks, total maybe being somewhere between 30-40 hours. Then I gave up. I thought it did nothing… but when i compared it to the case (indoors), there was quite a big difference!
There is talk how it makes the plastic brittle, aging it faster and turning it to dust etc… But if this is not happening over weeks, months or a year, I really do not care. If the sunlight would break things in short time, there would be serious warning about it to consumers.
FinApple 2019 (c)
Apple II Forever.