Not disagreeing on quality auto paint, but I've also had rustoleum on a tractor that sits outside 24/7 unprotected. Been about 10 years and the only damage is fading from the sun and scratches where I ran into something. A gallon of their rusty metal primer and then their topcoat is all I used after a power wash with some degreaser.
Again, that's exposed 24/7 to all the elements that northern MN throws at it.