In order to complete this article I used a scrap piece of end grain balsa deck structure. I wanted to use a scrap piece so that I could make cut-a-ways to show the inner workings and what actually happens on the "inside" of the deck.
Here I have drilled three holes; one for the #115 bit, one for the #654 and one that will remain as 1/4" with no epoxy fill.
The center hole, as you can see, has been counter sunk. This was originally a 1/4" hole that I over sized to 5/16" to make the #115 bit fit into it. Over sizing holes in fiberglass can often result in an "oops" and some serious deck chipping.
So here's the trick to over sizing holes:
#1 Pre-countersink/chamfer the hole to just beyond the width of the drill you will use for the over sizing, in this case 5/16". If your countersink bit is new and sharp get in the habit of using reverse, not forward, when countersinking fiberglass. Forward will remove too much material too fast if you are not careful.
#2 Place your brad point drill in the chamfered hole and use reverse. You will cut through the top skin only and have a nice clean hole. You do however need a brad point drill set. You won't find them at Home Depot and will find them a Rockler or on line. This is the quickest and cleanest way to bore the top skin without the drill wandering or causing serious gelcoat chipping!