That's a great question! For a trash can that's constantly exposed to sun, rain, and general wear-and-tear, you want a finish that's a true warrior. Based on durability alone, powder coating is generally considered the top choice for high-use outdoor metal trash cans.
Here’s why: It's not a liquid paint. Instead, a dry powder is electrostatically applied and then baked on, creating a much thicker, harder, and more uniform shield than traditional paint. This makes it incredibly resistant to chipping, scratching, fading, and rust. You'll often find premium metal cans with this finish.
A very close second, and arguably even more durable in terms of pure rust prevention, is a galvanized steel finish. This involves coating the steel in a layer of zinc, which sacrificially protects the metal underneath. You see this on industrial bins. It's extremely long-lasting but often has a utilitarian, silvery-gray "industrial" look that may not suit every backyard.
For plastic bins, look for UV-inhibited polyethylene or similar resins. The durability is molded into the material itself, preventing fading and brittleness.
To summarize: For maximum durability on a metal can, seek out a thick, baked-on powder coat. Avoid thin, spray-painted finishes, as they will chip and rust quickly. A good powder-coated bin, while a higher initial investment, will look better and last for many years without constant maintenance.