Q. I got halfway through a painting, only to discover that my masking fluid wont rub off! Why would this happen, and how can I remove it without ruining my painting?
A. Masking fluid, which is used to shield portions of white watercolor paper from colorants, was traditionally made of rubber cement. Today, masking fluids come in a range of materials and strengths, from those that provide 100 percent resistance to semipermeable films that allow color washes to adhere to the dried masking layer. A few masking fluids are classified as permanent. If your masking fluid is labeled as such, its likely that you wont be able to remove it.
Three ways to safeguard against masking problems are choosing the right product, preparing the surface correctly, and testing the product in advance. Read the product label first so you understand what youre purchasing. The instructions for many masking fluids indicate if the products are completely removable or have some degree of permanence.
Keep in mind that the way you prepare the surface can affect the properties of masking fluids. Just as watercolor applied to damp paper soaks into the surface, masking fluid behaves in a similar fashion, rather than sitting on the papers surface as it should. Many product manufacturers caution the user to apply the material on dry paper, as wet paper softens the sizing and speeds absorption.
Testing products is the key to avoiding such accidents. Label scraps of watercolor paper with surface preparation, masking fluid brand and the date of the test. Prepare several samples and try to remove the fluid in timed intervals to see if the mask still comes off after a week, a month, several months, etc.
As for correcting the problem at hand, you could create samples to reconstruct the situation and, before tackling the piece in question, try out the solutions on these scrap pieces.
You may also want to experiment (outdoors) with a solvent like acetone to dissolve (and then blot) the masking product, or you could use a knife to abrade the masking fluid from the surface. Finally, you might try using white gouache on the masked area so that color can be applied on top of it.