The two most common paint types for industry standards and artists alike are oil-based paint and water-based paint, as each type has distinctive properties that make them favourable in specific situations. It is possible to use the two products in combination, but as a general rule you shouldn't apply water-based paint over an existing coat of oil paint. The main differences between the two materials are the rate in which they dry and the manner in which the paint hardens.
Properties of Oil-Based Paint
Oil-based paint uses resins or oils, such as linseed oil, as a medium and hardens over time. Oil paint is known for maintaining the texture with which it is applied, which makes it ideal for some artists. Oil-based paint requires much more time to harden than water-based paint. Until the 1970s oil-based paint was the universal standard for homeowners, which is why problems tend to occur as people attempt to repaint with water-based mediums.
Properties of Water-Based Paint
Water-based paint consists of a plastic resin as a medium. Acrylic resin is a typical component of water-based paints and is known as a strong alternative to oil-based paint for artists. As it hardens, water-based paint generally remains flexible and is the more versatile option in many situations.
As a general rule, painters never use water-based paints such as acrylics to create layers over oil-based paint. Fresh layers of water-based paints will bond with the pre-existing layers but, as oil paint dries at a significantly slower rate, the bond will be corrupted as the oil-based paint contracts over time. The layers of water-based paint will become cracked or deformed, altering the appearance of a painting.
Possible Combinations for Artists
It is, however, possible and sometimes recommended to use water-based paint such as acrylic as a method of underpainting. Acrylics are perfect for doing a base layer of paint on material such as canvas, because the paint does dry much faster than oils, and because it is known for its smoother quality. There are no problems with using oil-based paint over existing layers of water-based paint, as long as the base layers are dry.
As the vast majority of houses were painted with oil-based paints preceding the 1970s, problems often occur as people try to apply new water-based mediums over the older layers. These problems can become worse as more layers are added, depending on the thickness of the original layers of oil-based paint. The difficulty is that, with climate changes, the surface beneath the paint will often expand or contract. Water-based paint, which is flexible due to its plastic/resin properties, will conform to the expansions or contractions. Oil-based paint, on the other hand, is much harder and more resistant to structural change and becomes more brittle over time. This is the reason for cracked paint on climate-exposed surfaces. When water-based paint is applied over layers of oil-based paint, a division develops between the mediums, and as contractions occur, the outer layers will lose their integrity. The outer layers of water-based paint may crack or deform and often peel away, leaving the resilient layer of oil-based paint behind. Preparation of the surface is generally a good idea, before applying new paint, and the removal of existing oil-based layers can be performed with an oil-diminishing solvent such as turpentine.