UV Gear's invisible porous ink is specially made for porous substrates such as paper, cardboard, textiles, currency notes, cloth, etc. These inks do not leave a film but can cause a visible mark depending on the type of material it is being applied. Always check a small amount to make sure you get the desired result. Appears clear but under a UVA blacklight it will glow blue. Ink is fast drying and can be used inside inkjet printers however we offer no guarantees for this application.