The fact of reyellowing is just a fact.It would be silly to assume twenty years of UV exposure is curd in a day and will never be affected by it again. They never made any claims that it was forever and do this at your own risks. I would ask Mcdermd tho as he has done this a bunch over the years and he may be able to confirm or have advice to keep it permanent. The solution may just be do it over again, its just a case and if you do use the computer at all, removing the hardware to clean it probably not a bad idea. I got a IIsi that has never seen sun, lived its entire life in a dark room and looked mint. It sits being useable on a counter now with normal lights, and soe occasional outdoor light passing by the window, and you can just tell its not as white/grey kinda color as it was. Its only been months. Do I really care? no,i like using it. Its peppy , functional and just all around neat.

I like the retrobright as it lets us see how they were but nothing, and I do mean nothing lasts forever.