Versatile acrylic paint lends itself to surfaces ranging from cloth to glass to leather, but artists most commonly use acrylics to create paintings on canvas. Various types of varnishes protect acrylic paintings from dirt, light, moisture and discolouration. Though acrylic paintings on canvas don't absolutely require a sealant, sealing your painting with varnish improves its durability and often makes the colours appear even more vivid.
Verify the paint is completely dry. Never use varnish to seal acrylic paint if it is even slightly wet.
Dust the surface of the dry paint gently with a clean cotton rag or paper towel.
Rinse a jar or cup with water to remove any dust particles.
Read the label on your varnish to find the correct water-to-varnish ratio.
Mix the varnish with water in the jar or cup. Use a different jar than the one you use to rinse paintbrushes to avoid getting wet paint in your varnish. Stir the varnish and water very slowly with your brush.
Rinse a synthetic paintbrush with water.
Lay a protective surface--such as a plank of wood or a piece of cardboard--on your work surface to protect against possible mishaps with the varnish.
Place your painting flat on the protective surface to prevent runs and drips.
Dip the brush into the varnish, lowering it about a one-third to one-quarter of the way down the length of the bristles.
Apply the varnish to the surface of the paint with smooth, even strokes. Run the brush across the painting in the same parallel direction for every stroke.
Inspect the paint in the light to spot any areas that you've missed, which will appear dull. Go over these areas gently with varnish.
Let the varnish dry overnight, or for the time period specified on its label.
Apply one or two more coats of varnish.
You can mix gloss, satin and matt polymer finishes to get just the right look for your painting. If you plan on photographing your painting, take pictures before sealing as glossy finishes can lead to glare. Many craft suppliers offer spray varnishes, which are easier to apply but generally do not offer results as durable as brushed-on sealants.
Work in a well-ventilated area and wear a mask as varnishes often produce heavy fumes. Do not attempt to paint over polymer varnish as doing so may ruin your painting.