My Kubota
L4400 is nearly 12 years old. Half it's life spent under a tarp, the other half in an open sided shed. Those were the only options. For the first few years I kept it very clean. Waxed large painted parts on occasion. Kept paint touched up.
This tractor is used hard when it gets used and now has its share of scratches and dings. I've also come to find that tractor paint jobs are not very good. And while my tractor has always been stored out of the rain and sun, there are lots of areas of powdery rust on the engine/chassis/trans areas (gray on a Kubota). Even under the shed it gets condensation and moisture. So between hard use, less than ideal storage situation and, in my opinion, low quality paint job it no longer looks new but still pretty good for a 12 year old machine.
The other factor is that the 'new' has worn off and no amount of cleaning and waxing makes it look new again. So in the last 4-5 years I have probably only waxed it once, actually washed it maybe 4 times. All the rest of the time it just gets hosed off.
I am an advocate of waxing a tractor that is not stored in a dry, completely enclosed environment simply for the protection it affords the paint. Birds now occasionally roost in my shed and poop on the tractor hood. When the hood is waxed the poop hoses off very easily.
Finally, a good looking tractor will get you more money if you decide to sell it.