Ok, so I set this on my weekend action plan but I've only properly just sorted it out. The screen-print rooms have spot varnish stuff readily available, which is fine. They also have UV reactive ink, but it shows up in sunlight. They told me that as long as I find invisible ink that is black light detectable and waterbased, it should eb fine to mix with the binder and work effectively.
http://www.blacklightworld.com/invisible%20ink.htm this website seems to stock this, but there's a few things I'm not sure about it, so I emailed them and I'm awaiting a response:
I was curious about your Black light, water based ink. How visible is it without a black light (I need it to be as invisible as possible)? Does it work under black light even when it's on darker surfaces? You say it's not permenant, but if I were to put it into a screen printing binder and use it on paper that wasn't to be exposed to water, would it last?
Thank you in advance.
Hopefully this goes well, otherwise I need to start thinking of a contingency plan.