Do breaks in the clearcoat spread? like dry rot?
I don't know if it's like dry rot or not, but I would assume that if moisture can get between the aluminum & the clearcoat it'll fail.
Our vintage trailer obviously has (actually had
, we've stripped it off as the first step towards polishing) a different clearcoat than what is used today, but it is pretty apparant that the coating had failed in stages. Some areas are more oxidized than others and in some areas, it radiates out in irregular sections from an initial pebble-sized ding. Most oxidation at the ding, then it gets less oxidized as the clearcoat failed over time & spread.
As far as preventing the "creeping crud" from spreading, I'll defer to someone with more experience...