Put a few turns of tape around a fiberglass rod at the point you are going to saw. The tape will keep the rod from splitting before you finish cutting through the entire piece.
Step 1: Tape Rod, Cut Good
You've got a fiberglass rod that needs cutting. Mark the rod where you want it cut, and roll some tape around the rod in that area. Remark the cut on the tape (if it matters). Now when you saw the rod it will cut more or less straight, with no premature and uneven spltting.
See the 4 picture sequence in this step.