Hairspray or lacquer them perhaps?

Don't panic! 🙂 The C64 keys/keyboards aren't precious, anyway.
The VC20 keyboard with its orange function keys is compatible and more popular among C64 fans.
There are also shiny new replacement caps.

This guy has a couple of awesome C64 videos that give an idea.
https://www.youtube.com/@RetroRecipes/search?query=C64

Edit: Hairspray is an interesting idea. We used that in school in art class.
Another idea is transparent acrylic paint, I often use that as the final layer after a paintjob.
However, sometimes it does make parts do yellow a bit. Perhaps some reaction with the other paint layer below, not sure.

Yes, some coating is an idea but I was hoping to be able to harden the existing paint. I also remember using hairspray to finish off school projects. Maybe it could help!

The other thing is that the white letters didn't come back as they should - despite the long bath (lack of UV, it was winter). So I'd rather not seal the yellow. But as you say, maybe I'll have to compromise on this one!

Hey, hairspray seems to have worked! 😁