Bill Houghton, Sebastopol, CA
A lot of porch balustrades are NOT built to be outside. If your balusters (spindles) are set in holes or grooves in the bottom railing, ensure that there's somewhere for the rainwater/snowmelt to go, rather than puddling. Drill holes in appropriate places, for instance (and prime the holes, so the end grain in the railing doesn't soak up water, thus leading to the railing rotting). That, and treatment of the bottom, will work together to add life.
I'm surprised that cedar is rotting that fast. Is there a lot of sapwood in the spindles?