Types of Paint
Paint usually falls into one of two categories: oil-based or water-based. Oil-based paint uses oil as the solvent. If it is called "alkyd," it uses synthetic oil instead of a natural oil such as linseed. Water-based paints use water as the solvent. Latex paint is a type of water-based paint that uses polyurethane as the binder in the paint. Acrylic paint is sometimes referred to as acrylic latex paint, but it uses acrylic as the binder, not latex.
Using Latex Paint
Latex paint has made some significant advances over the past 30 years and is quickly surpassing oil-based paints in use on exterior surfaces. Latex resists fading, yellowing and chalking. It spreads easier and dries quickly. Best of all, it cleans up with water, which means it doesn't need any chemical solvents. The one problem with using latex paint on exterior doors is that latex paint can bond with the rubber-like material that some weather-stripping products are made from. This can cause the paint to peel on areas of the door that come in contact with the weather stripping.
Using Oil-Based Paint
Oil-based paint has been the traditional favorite for exterior surfaces. It washes well and resists markings. Oil-based paints will bond to dirtier and weathered surfaces better than latex paint, because it creates a strong initial bond. A major drawback is that oil-based paints are high in volatile organic compounds (VOCs) which evaporate into the air and cause pollution. Oil-based paint is considered a toxic chemical by the U.S. Environmental Protection Agency.
Mixing Oil and Latex
It's well-known that oil and water do not mix. With paints, that's only half true. While you should never use oil-based paints over latex paint, you can use latex paint over oil-based paint. Oil-based paints are very rigid, latex paint is more flexible. The latex paint will bend as the door expands and contracts with temperature and weather conditions. If you apply an oil-based paint over it, the oil-based paint won't be able to flex as much as the latex paint and will crack, flake and peel. If your door is painted with latex paint and you want to change to oil-based, you will need to remove the latex paint before you apply the oil-based paint.