It really depends on how much you want to spend and what kind of quality finish you want to get.
If you're in the states check out www.southernpolyurethanes.com
, they sell top quality polyurethane clear coats.
I've used them myself for a number of years in the auto refinish industry and have done a couple of bicycles with this
material. My current bike is a Cervelo RS that I sanded down to raw carbon in some places, applied epoxy primer to it, then PPG Global basecoat and SPI production clearcoat, the finish is great and very durable, mind you I must have added 100 grams in the process.
For a raw carbon frame I wouldn't think twice about using straight SPI clear on it, you will need a least a small compressor and a mini gun to spray it, but it might be worth the expense if you do it right, and trust me, chances are that you would do it again on a number of different cycling parts, so it might be worth it down the line.
Check out Bohemian bikes online, he uses SPI exclusively on his bikes as well.
Just my $ .02 from working in the auto body industry and refinishing a couple of bikes and parts.