Masking is a technique used to achieve clear, crisp and clean lines in your painting. Because of the water-based nature of watercolor and acrylic paints, any type of oil-based compound actually resists the paint that is applied over it, effectively masking the canvas or paint underneath. Oil paint is also oil-based, however, so this technique does not work as a resist. Instead, the key to masking in oil painting is effectively blocking the paint from touching the layer underneath.
Masking tape is an inexpensive and easy way to mask your canvas or underpainting while oil painting. Masking tape is designed to be lightly adhesive and easily removable. Purchase your masking tape at an art supply or home improvement store to ensure it will not leave residue or damage your painting. Do not use duct tape or electrical tape. Apply the masking tape to the areas you want to mask, and paint the surrounding areas. This will leave the canvas or paint underneath the paint its original color. Some people prefer to leave the tape on until the painting is dry, but this isn't necessary as long as you are careful to maintain the lines while you continue to work.
Masking fluid is designed for use with watercolor paints. It is traditionally brushed on in thin coats, and hardens into a solid mask. It resists the watercolors, and protects the canvas or paint underneath. Masking fluid also works with oil paints, although it must be applied in thicker coats and allowed to dry before painting to ensure it adheres to the canvas and does not allow the paint to bleed underneath the edges. It should not be used on areas which have already been painted, but on unpainted canvas only. Masking fluid should be completely removed from the canvas after the paint is applied. Masking fluids that are designed to leave on the painting permanently will not work with oil paint. Removal requires gently "erasing" the fluid with a rubber eraser or scraping it away with a palette knife.
Melted wax can be used to mask areas of the canvas when oil painting. Wax adheres to the fibers of the canvas, and will keep the oil paint from spreading into the masked area. After painting, it is important to use a slightly abrasive cloth to remove the wax. Wax can be difficult to completely remove, and should not be used on previously painted areas because removal can damage the underpainting. Oil paint should not be applied over even a small amount of wax and left permanently because the oil paint will cure before the wax dries out and this may damage your work.
Rubber Cement and Other Compounds
Each artist develops his own preferred technique for masking. Some artists will use household compounds like rubber cement, glue or silicon caulk for masking. The results from using these types of chemicals on canvas differ, and they should be used very cautiously on previously painted areas. In some cases, they are also difficult to remove and may cause problems with your painting in the future. Experiment with different types of compounds to find out what works best for your painting.