Once dry, acrylic paint forms a strong enough surface that there’s no need to varnish it for protection. It can still be a good precaution to take, if the painting will be hung in a harsh environment that includes smoke and dust. Be sure that your canvas is drybefore varnishing it. It’s wise to wait three to five days for an average painting and a week or two if it has thicker coats (impastos.)
Most varnishes sold for oil paintings can also be used for acrylics, but you may still want to check the manufacturer’s recommendations. Apply them with a brush using wide, steady strokes. Most types of varnish are available in different finishes, including matte, glossy, satin, etc.
Aerosol varnishes are particularly easy to use for acrylic paintings. They’re normally applied vertically, in thin layers, and used in a well-ventilated space.
Varnishes for oil paint
When you varnish an oil painting, it smoothes out the finish on the work and protects it from damage. Varnish is a stronger coating than the layer of dried paint, which remains fragile even when it’s hardened. If you varnish a painting before it’s fully dry, a number of problems may occur: the varnish may crack, or soak into the paint, which makes it impossible to remove, or make the painting permanently sticky. Generally, you should wait six months for a painting without too many thick areas of paint, and up to a year otherwise.
Varnish should be applied with a soft, fairly wide brush, with the canvas laid flat. Use large brushstrokes and keep them parallel to one side of the canvas. When the first coat is dry, apply a second one perpendicular to the first.
Varnishes for watercolour paint
Varnish is not really necessary for watercolours, but it can preserve colours and protect your work from dust. Generally, varnishes for watercolour are applied in one or two thin coats to create a glossier surface, or to give a uniform finish when different types of paint have been used on the same work. Spray fixers are also an option.