Printers print, papermills manufacturer the paper. The watermark could be in the paper.
Correct. It is added to the paper during its raw production run. The printer would have nothing to do with it.
Speculation;
https://patents.google.com/patent/WO2004003853A2/en
Abstract “Machine-readable light-colored or invisible, but luminescent, postage-evidencing symbology and dark, readable other postage information are printed, preferably in a single print operation, to provide large amounts of information without diminishing read rates....In a preferred form, the images are printed using conventional ink jet printers using water-based inks. In one embodiment of this type, the images are printed with a conventional two-cartridge color ink jet printer by printing postage-evidencing symbology with a single-color ink and printing the visible dark image as a composite dark color from a standard multi-color cartridge.”...Snip
Note** that USPS generally uses a Black ink for mail routing bar codes, although I have seen luminescent yellow ink on envelopes that is sort of visible.
and:
http://www.digitex.is.it/en/servizi/inchiostro-invisibile-e-sistema-di-rilevamento/
...snip
“- High reliability: our InvInk system works perfectly both on the side of creation of the invisible references, that is made by a spray systems controlled by a microprocessor, and for what concern the spots automatic detection, always reliable and possible at any speed....
...Snip...
- Flexible and configurable: The system for invisible tagging and automatic detect, in its standard configuration, allows many simple regulations to modify the spot size and shape and the amount of ink used for each spot .
....snip. (More at at link.
(**Note as discussed their system is designed for high speed labeling and process and control operations.)
So, It is Possible, however:
***Objections to 1):
a) Requires intervention by printers of ballots, OR, after receipt by the State or County officials in charge of sending out the mail in ballots. (Invisible ink coding after state or county receipt of blank ballots.)
b) could a ballot reading machine distinguish between the visible ink voter choice and invisible Biden choice and choose the invisible over visible if both barcodes were next to or even on top of each other on a ballot?? This would be a software review issue IF they find invisible ink markings.
2) Mail in Ballots envelopes sent to Republicans have something to mark them segregate them to assist in special handling (Spoliation.) Could be invisible or visible ink? (Could be just a pencil mark on outside of envelope I suppose if info is relayed to counting centers.)
Aid from someone at the Postal service operating the high speed sorting machines? Pubbie ballots get their own Tray.
(Note this story: https://www.washingtonpost.com/business/2020/08/20/postal-service-mail-sorters-removals)
At the counting center, once the ballot is separated from the envelopes the envelopes and evidence are then destroyed.
****Objections to 2) Again possible, but more steps and more people involved. (Entity issuing the Mail ins, collusion with Postal Worker in Sorting Facilities, relay and special handling of the trays.
CONCLUSION: If Trump’s lawyers have access to ballot (or envelopes) then the ballots should be checked for some form of invisible ink coding to rule out the possibility of manipulation. (If they get a lot of resistance to checking for invisible inks this they should definitely check!)