Dr. Ph. Martin's Bleedproof White is an opaque watercolor for covering dye colors, markers, and designer colours.
The 30ml bottle is concentrated so you will need to dilute with water if you are planning to use it with a dipping pen.
Diluted with water it is the most amazing white ink for using with pointed dipping pens for an elegant look on darker paper. I have also used it on glass with a pointed pen and it writes beautifully.