I bought an original "breadbin" model of the Commodore 64 earlier this month. But, I didn't unbox it until last night after buying a custom / modern power supply for it to avoid some of the bad problems old Commodore power supply units have. Although it powers up and runs perfect, including with great sound, I found some keyboard issues and decided to try to fix, replace or clean up the keyboard.
In the end, the old machine had what I'm going to guess are coffee or cola stains and muck in the keyboard. I was able to clean it up and the keys no longer stuck -- so nearly good as new!
With that said, I must add -- scrubbing someone else's coffee or soda from 40 years ago is nasty!