HP 41 DISPLAY cleaning

[aurelio]

I don't remember if there's already a discussion about this matter, but I wonder if is it possible to do something to "renew" or to clean the display of the fullnuts; mostly they have a yellowed look I don't know if from the smoke of users or just due to a natural process.
just once I disassembled one of them trying to repair the keyboard, but I was so worried in unsoldering /solder again the contacts and restoring the calculator that I forgot to give a clean to the trasparent cover to check if the problem was located there..... somebody knows the secret and succeded in doing it?