So, this is like the self-destructing text messages, only on paper: "This invitation to a free screening of the latest Harry Potter film was sent by Coca-Cola to executives fo important Spanish companies. You had to rub the surface with a wet cloth to see the text. After a while it would disappear again." (McCann-Erickson Spain, 2005 Silver Gold Medal at New York Festival.)
And while we are at it, here's where you can buy "black light" paint, that is paint visible in black light but transparent otherwise.