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 ALWAYS use reverse NOT forward when countersinking fiberglass!!! Forward will remove too much material too fast!

----------

#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!