Why do some artists prefer oil and some prefer oil as mediums? Some even combine the both! The old adage 'oil and water don't mix', is very applicable to this issue. From my point of view the 2 mediums should never be mixed, even painting with oil on top of acrylic is questionable from an art conservation standpoint. At least that is what I have read. I know one always hears artist say "but I have been doing it for years and still have some of those paintings". Well here is the catch, over time maybe lots of time maybe not, paintings can de-laminate. That means they start to come apart, layers my separate, crack or wrinkle. In some cases paintings can even become unstuck from the surface they are painted on. Mixing mediums is in a way asking for it!

Oil and water paints have different chemistry and different properties. Oil actually doesn't dry, it cures. An actual chemical change takes place over time called oxidation. At first it is dry to the touch, but that is just the result of mediums and solvents used in the painting process drying. The oxidation part happens over weeks and sometimes months or years. It is when the paint particles react to the oxygen in the air and create a tough film. It's always best for an oil painting artist to wait 6 months to a year before varnishing their work. If varnished too soon the varnish may dry faster than the paint underneath and cause cracking on the surface. Yes there is a product that can be used on paintings that are not 'fully cured'. However I'm not sure it has stood the test of time. In the literature I have read there are different opinions regarding its use.

Sentries in Perspective-acrylic

Acrylic paint actually dries through a process of evaporation, the water dries first and as it does the pigment particles which are suspended in the paint move closer together. This is called 'polymerization' and  similar to oil paintings the surface may feel dry before the layers underneath have fully dried. I read one article which said acrylic paintings may actually never be fully dry. I have no idea I'm not a painting conservator, but I highly respect that discipline and find their research and advice invaluable. Surprisingly one still has to wait a bit before varnishing. It is recommended that one wait 1 to 3 weeks before varnishing depending on environmental factors such as humidity and the thickness of the paint. Also recommended for acrylic paint is an 'isolation coat' between the painting and the final varnish. I appreciate both mediums and have taken the time to understand the positive and negative aspects of both. Not only their characteristics and handling properties but also the relative 'safe-practices' recommended by experts.