Q. Sometimes I get a strange bubbly effect when painting wet-into-wet with watercolors. The bubbling eventually goes away, but the painting isn’t smooth until it’s dry. Should I worry about these bubbles? Also, are they related to the fact that I don’t remove the sizing from my paper?
A. I’ll begin with a bit of a scientific explanation: Commercial watercolors generally contain a proportion of surfactant in the vehicle. The vehicle is the total liquid content of the paint, including the binder, which is the adhesive element. A surfactant allows the dispersion of pigments that normally resist being dispersed (such as many organic pigments) into the vehicle. Since a surfactant is essentially a soap, the bubbling effect could result from an excess of surfactant in whatever brand of paint you’re using. If you scrub the paints on, then this bubbling will be noticeably enhanced.
I wouldn’t worry too much about the bubbles as long as they go away. But I would worry a bit if this effect is produced without a lot of manipulation of your paints during application. If so, it’s possible that your paints contain more surfactant than is necessary, which can produce a film of dried paint that’s soft and easily damaged.
Finally, there’s a small possibility that the bubbling you’ve gotten is related to the size in the paper you’re using. Scrubbing the papers with a paint-loaded brush, for instance, could theoretically cause some foaming of the size. But watercolor papers are sized in order to prevent the uncontrolled spreading and feathering of applied paint, so unless you’re looking for unusual pictorial effects, you should not remove the sizing.
Mark Gottsegen is an associate professor of art at the University of North Carolina at Greensboro and the author of The Painters Handbook (Watson-Guptill).