I’ve never encountered this issue before, usually my fingernail was good enough. But if it doesn’t work for you, try using a cutter blade, just the blade! Place it flat, sharp side against a corner of the protector and push carefully

Np, hope you enjoy the unusual protection

So it’s a kind of lubricant, surely water based. I’m not a big fan of the silicone screen protector but it seems like a good option for the shop, as there are so many devices of different sizes instead of having a huge stock of standard ones. It’s not a printer, the machine has a database for all the sizes and it cuts the silicone sheet accordingly

Maybe you confused water with glue, it’s common to spread water so you can move and place the silicone layer correctly