This door was stained. We brushed a concentrate stain on and when it was dry we brushed a clear coat of an oil polyurethane. Thanks for asking!

Which Brand do you recommend on the oil polyurethane?

In my experience an exterior oil polyurethane is an exterior oil polyurethane. The key is always doing it. Keep an eye on how it weathers each year and recoat it as you feel it needs. On exterior wood surfaces I won't use anything but spar urethane. It's a polyurethane that has way more flexibility. So when the wood expands and contracts, the coating will too. However sun exposure slowly makes the coating less flexible and more brittle. Keep a fresh coat on your exterior wood surface to prevent the coating from peeling.