Going to install 2 strap brackets for Bimini with #10 screws. Best I can figure final drill size is 5/32. What technique for drilling without cracks.

thanks

Easiest methods I’ve found are don’t use an extremely sharp bit, go in reverse to burn through the first layers, lightly counter sink the hole before you run the screw in. If you don’t have a counter sink or are lazy to change your bit you can also use your phillips screw tip as a small counter sink haha

Quoted for truth...THIS is the best way.

X's 3
And use a very light touch !

Do not run the screw into the glass without an appropriate counter sink. Best done with a stone so as to not create any rough / sharp edges. Just want a smooth bevel, and do not over tighten the screw and pull the gell from the glass substrate.

^^^^ This, plus one more hint based on years of experience. Normally, when sizing a hole for wood or something similar, one goes slightly smaller than the root diameter of the screw when sizing the drill bit. For fiberglass (topped with gelcoat), it's a bit different. Size the drill bit exactly for the root diameter or even a bit bigger. Although this theoretically reduces the strength of the hold down, it also prevents stress cracks.

i always put down masking tape on the fiberglass before drilling, reduces cracks/chunks.
remove after drilled.

I totally agree with this...A masking tape on the fiberglass before drilling does the trick.

Why dont you thru bolt it?

Masking tape and then counter sink