Originally Posted by Rogue Wave
for a big circular hole buy, hire or borrow a bi metal hole cutter.
I'm yet to find a bimetal hole cutter that is the right size for normal gauges which means cutting undersize and loads of sanding. Also big cutters are bloody expensive and want to turn slower than the typical variable handheld drill wants to go.
On a current project I used one of these adjustable hole cutter jobbies. The GRP does blunt the blade fairly quick, but since you can set exactly the right size, it's very convenient, and since there's only a single cutting edge, it's easy to sharpen again on a stone (or angel grinder)