This image shows the 5/16" over sized hole, and the original 1/4" hole on the left. The new bigger hole is just as clean even though there was no "center point" to start the drill with.
At this point I have removed the core with the Dremel and you can see the cavity it created if you blow up the photo and look close.
When using the Dremel to rout the core away always make it a practice to insert the head of the bit into the deck before turning it on. Once turned on use the bits shaft as a guide and work your way around the hole moving up and down and around the circumference of the deck hole to clean the core material from the the skins and to make a nice, even, round cavity for the epoxy to seal the deck.. Make a few passes and suck any dust out with a shop vac. It takes all of about 10-20 seconds, per hole, once prepped and ready for it..