I haven't seen or heard anything about supposed watermarks on the ballots...

UV light can show pencil/ink marks erasures and other tampering.

https://www.jstor.org/stable/1141001?seq=1

EDIT:

Here's a better explanation of forensic document examination:

https://forensicfield.blog/application-of-uv-light-in-questioned-document-examination/

....

However, the UV light might also be a sterilization process for C-19 protection.

I haven't seen or heard anything about supposed watermarks on the ballots...

UV light can show pencil/ink marks erasures and other tampering.

https://www.jstor.org/stable/1141001?seq=1

....

However, the UV light might also be a sterilization process for C-19 protection.