I am a DIY who is redoing a bathroom countertop. I am using 5/8 exterior plywood and plan to use thinset mortar between the plywood and 1/2" Hardibacker board(In case you are wondering, I used the 1/2" instead of 1/4" because I needed the solidity and needed to raise my sink up a bit to the level of a backsplash). I used a spiral saw to cut out the shape of my sink on the plywood, but found that even with a so-called cement board bit the spiral saw will not cut the Hardibacker board. The instructions say to use a circular saw with a carbide blade to cut the Hardibacker, and I have one, but am not sure how to cut such a round hole with a circular saw. Do I just make a zillion straight cuts and eventually get rid of all the material bit by bit? Seems like there should be a better way. Help!