Hello,
I’m in the process of installing new deck handrails and went with the pre-cut for spindles type of railing. (They have square holes routered out to seat the spindle in) In theory I figured it would simply be a matter of installing the pre-cut rail on top and flip one over for the bottom to seat the spindles in both top and bottom. Once I actually saw the completed rail I became concerned about the fact that the bottom pieces have concave, over-sized spindle holes that are somewhat exposed to the elements. Won’t these collect water? Is that a problem? Are these pre cut handrails for use on the top only? Am I needlessly worried? Also, do the spindles need to be nailed into place or do the cut-outs keep them secure enough? Thanks for any helpful hints and advice. As I’m sure you’ve gathered, I’m a novice when it comes to construction of any type.

You have discovered a potential problem, water will collect. That top rail is only for the top. Most spindles are mounted a bit further out toward the edge and get nailed to the side of the perimeter board. Not sure how to salvage what you have since I would not have done it that way.