While Dr Oh Martin's bleedproof white has many things it is excellent for, my favorite is writing with it. You do need to thin it, but a few drops of water suffices. Dip you pen and do a few test strokes. It will write a nice opaque white line. The more you thin it the less opaque. You can even write with it all the way down to almost transparency. Highly versatile, provides lots of flexibility for your art.