Rod, it really depends on what kind of cut you are making. A thin cut-off wheel is fine for straight cuts But for curves, it is another story. I frequently use a saber saw for some types of cuts. Drill a 1/4" hole at the end of each cut, add masking tape on the surface to protect the gel coat from getting scratched by the saber saw and then cut using a thin blade. For large-radius corners, I use a hole saw and then cut between them with a saber saw or an abrasive cut-off wheel. Finish each cut edge with silicon carbide abrasive paper to leave a smooth, slightly rounded edge. NEVER use sharp corners when cutting out holes in metal or in fiberglass. Those will be where cracks develop. Look up the problems with the early De Havilland Comet jet airliner.
