Water-based paints, such as latex and acrylics, are desirable because they are much safer and dry much faster than oil-based paints and do not require toxic solvents for cleanup. While water-based paints are durable, they last longer if properly protected with a finish, such as varnish. However, not all varnishes are suitable for a project.
Oil-based varnishes are formed from mixtures of oil and resin, and cure by evaporation. They dry slowly, and are often used as an alternative to lacquer, which is somewhat dangerous and often not suitable for an amateur or hobbyist. There are three basic types of oil based varnish. Phenolic resin is made from a mixture of phenol, which is a plastic, and formaldehyde. Alkyd resin is made from a mixture of polyester resin, alcohol and acid and is the least expensive of the oil-based varnishes. Polyurethane is also sometimes considered an oil-based varnish, and it is noted for its high durability.
Acrylic and Latex
Although there are other water-based paints, such as tempera and watercolor, acrylics and latex paints are the types most commonly used for painting homes and furniture. Acrylic and latex paints are both formed from synthetic polymers and can be thinned with water. However, there are different types of polymers that are used in latex and acrylic. Acrylic polymers are the highest quality type, and a paint that is 100 percent acrylic polymer is a high-quality paint. Latex house paints are typically 20 percent acrylic and 80 percent vinyl.
Mixing Oil and Water
Oil based products and water based products typically do not work well together. In particular, painting with acrylics over oils usually has poor results. This is mainly because the drying times of water-based products and oil-based products are different. As an example, acrylic paints should not be used over oil-based paints. An oil paint may feel dry, but actually continues to cure over several years. It therefore forms a weak bond with acrylic paint if the paint is applied on top of the oil paint. However, acrylic paint dries very quickly and is somewhat porous, so an oil-based paint can be applied over the acrylic.
Using Oil-Based Varnish
An oil-based varnish can be applied over completely dry acrylic paint with little difficulty. The polyurethane type of varnish is usually the best choice for this. However, oil-based varnishes typically have a natural amber tint and often become more yellow over time. This can distort the colors of the item you have painted. If possible, it is always best to use an acrylic-based varnish for water-based paints and oil-based varnish for oil-based paints to ensure good results.