Since new I've routinely cleaned my Michelin XZA tires and periodically applied various protectant products including 303. The tires have never been covered. The rig seldom is stationary for more than a few weeks as we routinely move several hundred miles at least once every month.
Anyway, after about 5 years my tires began to develop minor cracks and now at 7 years they have some significant cracks that per Michelin's guidance require replacement. Six new tires will be installed in 3 days!
Bad news is the huge cost on a fixed retirement income
Anyway my rig has spent a lot of time in the southwest (San Diego, Las Vegas, Arizona, etc) where the temps tend to be much higher and the sun is much stronger than other parts of the country. So I tend to agree with Dieselclacker as the location (temps and amount of sunshine or UV) seems to have a significant impact on the development of cracks.
It was interesting that while shopping for new tires more than one dealer strongly suggested looking for an alternatvie to the Michelins as in their opinion the Michelins seemed to crack much worse than any other brand. Given I drive only about 15K miles per year, I most likely will never wear out the tires before cracks develop, regardless of brand.
Anyway I've elected to go with Yokohamo RY023 tires. The RY023's are proven tires and more than meet my size and weight requirements. Hopefully future cracks will be minimal.