We've all tried writing with lemon juice (a.k.a. invisible ink). It's a mess. And the main draw back is definitely efficiency.
Spy work requires extensive (and sometimes wordy) communication.
Enter: the printer mod. We were super impressed when our friend Loup sent us this video on printing with invisible ink!
The ultraviolet ink is the most sophisticated of the invisible breed.
Some alternatives we found were urine, vinegar, wine, honey, lemon juice, milk and onion juice.
For the very adventurous...consider British Secret Intelligence Service chief, Sir George Mansfield Smith-Cumming's favorite covert ink: semen. As an endorsement of its official use, he proclaimed to his spies, "Every man carries his own stylo".
WonderHowTo is not responsible for semen damage to your Epson.