Varnish is essential for the protection of oil paintings.
Oil paints consist of pigments mixed with an oil binder, such as linseed oil, which can take a long time to dry and harden. Once the painting is dry, the oil binder can still be vulnerable to environmental factors such as light, heat, and humidity, which can cause the paint to crack, fade, or discolor over time.
Varnish is a clear, protective coating that is applied over the dried oil paint to help protect it from these environmental factors. It creates a barrier between the paint and the outside world, helping to prevent damage and deterioration. Varnish also enhances the appearance of the painting by bringing out the colors and creating a more even sheen.
While varnish can also be used on other types of paintings, such as watercolor or gouache, it is not as essential because these types of paints do not have the same vulnerability to environmental factors as oil paints. However, it is still recommended to use varnish on these types of paintings if you want to protect them and enhance their appearance.