This is a question about types of paints and their best applications, but more specifically about the best type of paint for an exterior door. Some context: I have a wood core exterior door that appears to be wrapped with a type of hard plastic (see photos). I found a can of paint in the garage that the last owner used to paint the metal door that preceded the current door. The can was specifically labeled as an enamel for exterior doors. Based on this, I assumed that there is indeed a special paint formulated to meet the demands of an exterior door, i.e. heavy use and traffic, movement of the painted material, exposure to the elements. Yet when I inquire with the paint department of a big box store I get puzzled looks and the reply is that most any exterior paint will work. Is this true? Or is there a best paint that will give me the best durability?
A high quality gloss finish paint will give you the best durability. Whether a water-based or alkyd paint is "better" is a long-standing point of contention that I will not address beyond my points below, except that to mention that thinners/solvents for oil paints seem to have gotten very expensive, along with all the other extra work when using oils.
- there are differences between "budget" paint and high quality paint. You don't mention color but for light colors, higher quality paints tend to have better hiding/coverage due to higher pigment (titanium dioxide) content. If a manufacturer elects not to reveal the pigment percentage on the can, one can often get an comparative idea by the weight compared to competing brands. They also tend to have higher-quality carrier (solvent) formulae as well as anti-fungal and flow-enhancing additives.
- for water based finishes you should choose acrylic paint, preferably 100% acrylic if available.
As always (and as likely mentioned by others) surface preparation and use of high quality applicators will make a huge difference regardless of the paint.
What you have looks to be a PVC clad door.
Most exterior paint will work on "most" exterior doors. However metal clad doors would benefit from paint formulated for metal, such as an oil based enamel.
In your situation, you need to look for a paint suitable for a PVC clad door. If you ask for paint for an exterior door, you will be presented with exterior grade paint. Be specific and ask for an exterior paint that will be suitable for a PVC clad door. You may be better served looking in a paint store rather than big box.
In any event you will need to thoroughly clean the door and lightly scuff it with a light pad. Whether or not you need primer will be under the advice of the paint store and the paint they recommend.