I fit my own tyres, the problem is obvious once you whip the old tyre off. As soon as the corrosion creeping under the paint reaches the bead, you've had it.
You can clean the rims back to bare metal and apply rimseal, but you then have to do this every time you change the tyre as the corrosion continues to creep under the remaining paint. It takes time to make a decent job that will last, it also makes tyres harder to remove with a mandraulic tyre changer.