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Slowly and slowly, they start to "let go" bile, but they have to "swim" for a few good days.

Unfortunately, bile often likes to come back over time ...

It's called acid reflux, and there are medications for it
But the paper does rub, including whatever is printed on it.
Actually, it could be varnished or laminated after printing, but I don't think it looks nice.

and this plastic reflux is probably better than any other medicine is white spray paint;)

As for stickers, when you cut them nicely and stick them easily, then it looks nice too: D

Everything rubs off, even the factory print on the keys, the vinyl foil varnished before sticking, however, has a chance to withstand a bit. I used to make a faceplate sticker in a rubber boots and I will say that after six months of use it works.

Ideally, I would be able to get a laser engraver, but that's beyond my reach.

It happens differently, depending on individual retro models, on own examples it is the Amiga 600 cork, e.g. it can turn yellow again after 1-1.5 years, but for example the clavicle with CPC 6128 is already the fourth year and then white. Similarly, Sam Coupe, I whitened the whole thing and "sticks" for 3 years. It's my first time with Timex, we'll wait and see

Do you shine with UV LEDs?

No, it just stands close to the heater and "works". During the day I put it to the window (from the inside, of course), because today I had a nice sun for a few hours and it even warmed up nicely. Now again at the radiator. From time to time, every few hours, I just come over and mix it with this bottle. And with the cap, be careful that it is ajar because it evaporates and the pressure can burst.

My grandfather had such a large cylinder in the basement and there was always something working there, but it was blocked with a stopper with a glass tube. However, I have never seen any keys in this cylinder

Wax, a lot of wax on the cork must be there

According to my experience, the full spectrum of sunlight helps during whitening.
Any ABC UV light wells will not give a better effect.