Simon, 5 kg's?? Whow! Never tought about that, but you are probably right. Thinking back to when I painted the last set of hulls, it required quite a lot of paint..
But if you are not going to paint it, what are you going to do? Go for the 'natural' carbon finish (which requires UV blocker in either the matrix or a clear finish with UV blocker)?

Yet another argument for boats buildt in wood with a clear finish. Or.. How much does 5 layers of clear UV block weight?