here you go

it is the watercolor-like brown splotches i'm talking about. the rest is sediment from a recent rainstorm.

It appears to be some kind of contamination - oil, grease, even a muddy spot that did not come completely clean before painting.

It looks like its coming from underneath to me. I don't really know though.

It was clean before painting. It was literally pure white.

Cedar sawdust (and other species) will make surface stains like that if it splashes up or blows onto wet wood. If the rails were cedar, seeing as how they were just installed, there could be some sawdust in the vicinity.
Casey

They were installed a year ago. They are fiberglass