Is Carbon good for mast struts?

depend if your trapezing and don't let go when you miss the hook....funny how for the first couple of times your mind thinks keeping hold is a good thing!

Originally posted by AlanH I was going to recycle broken tiller ext as a strut

Very much doubt that would be man enough.

Thanks for input guys. I had a feeling there might be some downside, and getting battered by a metal clew doesn't sound nice. Just weighed the alloy strut, its only about 300g, so not much weight saving on offer. I was going to recycle broken tiller ext as a strut so wouldn't cost much, but think I'd best stay with the old tech here. Pimping is a bit of a luxury.

Black paint?

Yep Jim is absolutely correct.
Resistance to buckling due to axial compression is a function of length of strut, Young's modulus and section inertia.
In an application where the part would be expected to operate in a post buckled state, then the ratio of zero axis and off axis fibres would need to be considered in particularly thin walled applications, but I suspect this will be fine in a little strut.
Just ensure that what you get fits the end pieces well, and that the load is taken by the strut and end pieces, not by any screws or rivets attaching the strut.
If using mech fasteners take care to drill appropriate guide holes to te fastener, else it is easy to damage the laminate.