When wood is cut across the grain, the end of the grain is exposed. This can be seen at the end of a board. The end grain of any wood gains and loses moisture more rapidly than the rest of the piece, which is why it needs to be protected. Keep in mind that wood will always assume the moisture and temperature of the surrounding environment, which is good. Problems arise when that change happens too rapidly.
This is why we recommend priming the end grain of your spindles, balusters, railing, and other millwork before installation. If you choose to have your millwork primed here in our shop, we spray a high quality oil based primer covering all surfaces, including the ends. We use a very slow curing primer which means the protective oils seep deeply into the wood grain before setting up. This means your millwork has the best possible protection against the elements and will ensure decades of trouble-free enjoyment.