Originally Posted by h22lude
You wouldn't want that for primary. You need some head room.
Not if you use a blow-off tube and don't over-fill it... You don't need a gallon plus of headroom in a primary. I wouldn't put much more than 5 gallons in it though. I would also use fermcap-s in it if using it as a primary.
With a little brain power, you can use it for a primary just fine. I've been using my two 5 gallon carboy's as primaries for several batches. I even used a 5 gallon glass carboy as the primary for a batch of mead (5 gallons in it).
You DO need to make sure it's the right kind of PET bottle though. If it's not, then don't use it.