aemeijers wrote:
-snip- that looks chemical, not heat-related. My
first thought was that whoever did the last stain job on rail got a glob on
wall, and wiped it down with the wrong solvent to clean it off. Even if the
solvent didn't cause immediate visible damage, it could have made the
plastic more sun-sensitive.

aem sends...

I'll vote for Doug Miller and aemeijers suggestion -- looks chemical and
likely caused by harsh solvent ingredient (like acetone) used to remove
deck stain overspray and damage took time to become apparent.