Cutting a perfectly round hole in a piece of finished fiberglass without damaging the gel-coated surface around the hole takes careful preparation and the right cutting tools.The thickness of the fiberglass will determine which cutting tool to use. For example, cutting a hole in a thin board can be accomplished with a spiral Dremel tool or heavier-duty Rotozip. Both power tools can be equipped with circular cutting attachments. Thicker materials may require a more powerful jigsaw or compressed-air-powered air saw.
Mask the area to be cut. If provided, use a template to scribe the correct circle size. Now mark the exact center and make an indentation with the punch at that spot. The indentation will keep your bit from skipping and prevent the sides of the drill bit from cracking the gel coat as it bores through the fiberglass. Don the safety glasses and mask before drilling the starter hole and make certain there is nothing directly below the hole area that might suffer damage as you drill the cutout. Start drilling with a 1/16-inch size bit, then increase the size to 1/8, 3/16, 1/4 and so on, until the hole is large enough to accept the spiral saw.
Cut out to the edge of the scribed circle; then, using the circular cutting attachment, slowly cut around the perimeter. Don't force the blade, and be careful not to over-rev the tool so it won't burn out the brushes. With the fiberglass plug removed, strip off the masking tape and examine the edges of the hole for scalloping or unevenness. If needed, sand the inside of the cut with 200 to 400 fine grit sandpaper until smooth. Remember, if an instrument is being installed, it usually has a lip that will cover any small imperfection around the edge of the hole.
Vacuum the work area thoroughly after completing the job. Fiberglass dust is unhealthy and is easily tracked away from the work space to where children and pets can get it on their hands and paws and into their systems. Finally, be sure to unplug all power tools and remove them from the job site before calling it quits. Congratulations on another job well done!