For best results and durability, we recommend Sherwin Williams Emerald Urethane Trim Enamel paint. It's great for both the interior & exterior sides of your door, is easy to apply, and is extremely durable. We've selected it because it's a paint that combines the uniform finish of an oil based paint with the application ease of a water based paint.

Acrylic latex paint (exterior grade) also can work well as it is fairly durable and also easy to work with (doesn't dry too quickly or too slowly).

Oil based paint can be used with sanding/priming, but fades faster and is not as durable for exterior applications and has tendency to crack if door expands/contracts (e.g., with a wood door)

You should paint your door every 4-5 years to protect the door from rot or rust and to keep your door looking its best, especially if you have high sun intensity/darker colors, which can fade more quickly. If you start to see rust spots on your door, we recommened to sand down to good steel, prime, and get a fresh new coat on ASAP for protection. Otherwise you risk the door rusting through, which typically requires the entire slab to be replaced.