Some faux finishes call for a varnish to protect the surface, and varnishing over dull paint can brighten it and add shine. Varnish can be used over latex paint if you use the right kind; however, using the wrong varnish can change the color of the paint. As a result, there are some techniques you should be aware of before starting a varnishing project.
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Assess Your Goals
The latex glazes and paints used for faux painting are quite washable, but varnishing offers an extra layer of protection. So, faux finishes often are varnished over, especially if the surface is going to take some use and abuse. The varnish also can serve to even out the gloss and intensify the colors.
If you"ve used a high-quality, durable latex or acrylic paint, varnishing over it won"t necessarily make it more durable. But if the paint used was lower quality, or if you simply want to increase the gloss, varnishing is the answer.
Use only interior varnishes over interior paints, and exterior varnishes outside. Bear in mind that exterior oil-based or marine spar varnishes have a tendency to yellow over time and will change the paint color.
Choose the Right Varnish
Virtually any type of varnish will work over latex paint except lacquer. Lacquer will wrinkle and ruin the finish.
Oil-based varnishes and polyurethanes can be used over latex paints. They provide the toughest finish, but will all yellow the underlying color to a certain degree. Avoid their use on white or very light colors unless you are trying to achieve an antiqued or aged effect. They also take longer to dry, so bear this in mind if you are doing multiple coats. You may only be able to paint one coat a day.
Water-based polyurethane works very well, dries faster and is non-yellowing. It is not quite as durable as oil-based varnish, but is still very resistant to scuffing and abrasion.
Water-based acrylic varnish also can be used over any type of latex paint. It is thinner than either oil or polyurethane, and may take an extra coat to achieve full hardness and shine. However, it is easy to apply and dries quickly. Beware: Acrylic varnishes can scare you at first. They go on milky-white, but rest assured they dry to a crystal-clear finish.
Prepare the Surface Correctly
There"s no need to sand or clean a freshly painted latex or faux-finished surface. Simply wait at least eight hours before applying the varnish and apply it as you would another coat of paint; just make sure your tools are completely clean.
For older, cured latex paint surfaces, either use a fine sanding sponge to lightly sand it down, or use a "liquid sander" (see Resources). Make sure the painted surface is perfectly clean and free of dust before varnishing.
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Never shake any type of varnish before application. This creates little bubbles that can give you an uneven sheen. Instead, stir it with a clean stir stick, then pour small amounts as needed into a clean container to work from.