Recently I purchased a new to me art tool. I bought Liquid Frisket from the Dick Blick store near me. It can also be purchased online and is available in other brands.
This is what Dick Blick says about frisket. Blick Liquid Frisket
is intended for use on watercolor papers, photos, and other
media where overlying applications of watercolors, gouache,
or ink are to be made.
Here is a very simple card that I made using Frisket as a mask with some watercolor. I applied the "hello" with a brush. I think it could be applied with a pen nib as well.
I know that rubber cement can be used as a mask with water color and other mediums, I've tried it and find this works much better for me. This is thinner and the product instructions say you can thin it a bit more with water. I didn't try that yet. It also says Frisket needs to be used on a dry surface so if you wanted to add a second color (masking out a bit of the blue for instance) I gather you have to be sure the first layer is completely dry. I will try that next time.
Have you ever used Frisket? It is always fun to add a new tool to art supplies.