Your best method to repair this is to remove all of the cracked material. You just have to be aware that when you start drilling into areas around existing holes and the riser pin, you never really know how much material may additional crack or come out.
More importantly, you should try to eliminate the cause for this happening. There are two main reasons that this occurs in bowling balls:
1) Edges of gripping holes not sanded or beveled
Many pro shop operators refuse to slightly bevel the edge of finger holes that grips are being installed in. I've never understood why they don't take the extra 10 seconds. It really does prevent the circular hairline cracking on the surface of the ball.
2) Using glue too close to the coverstock material
Some fast bonding glues can actually attack the coverstock chemicals making them much more brittle than normal. When installing finger grips, always make sure to glue below the coverstock/outer core seperation line.
Good luck!
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Jeff Ussery
Powerhouse Training Clinics
www.proshoptraining.com