There are three primary reasons for drilling.

Reasons for Drilling

1. Long cracks. Drilling the end of a crack over 3” long relieves some of the stress that can cause the crack to run during the filling process. After it is filled with resin and cured, the drill hole at the end of a crack adds stability and helps prevent future crack outs. It’s very important to select a bur that matches up with the point size and shape of the slide hammer or spring hammer. The Delta Kits 26020 Tapered Carbide Bur is perfectly suited for this purpose.
2. Sub-surface damage with no surface chip by which to access the damage. In this case, a hole needs to be drilled through the surface of the glass and into the airspace below. A very small bur is well suited for this purpose. Cosmetically, a smaller hole will look better after the repair has been completed. My preference is the Delta Kits 26030 Round Carbide Bur due to small size, durability and skip-free round tip but we offer four other various shaped carbide burs that are as small or smaller depending on your preference.
3. Capping a large pit. If the pit is too large to completely cover with the injector end seal the easiest solution is to cap the pit with pit filler and scrape flat, then drill a small hole through the cap to access the air space below. I really like the Delta Kits 26040 Pear Carbide Bur. It’s not quite as durable as the aforementioned burs but it drills a tiny hole that is barely noticeable and because I’m only drilling through the resin, durability isn’t an issue.