Typically installers will use a latex caulk for fiberglass porch columns. It is known as the painter’s caulk as it accepts primer/paint very well. If you aren’t planning to paint over the caulk silicone caulks fare better as they will expand and contract with the column and separate less. We also recommend a polyurethane caulk from MasterSeal that we have had over a decade of success with. You can read more about caulks at the link in the previous question.