http://forum.woodenboat.com/showthread.php?1958-Graphite-epoxy

When I did my first boat with carbon filled epoxy it was for the faux lines I routered into the sliding hatch and lazarette hatch. It was in the late '70's and I just ran carbon sticks through a pencil sharpener, shook out the fines and used those in the epoxy. I saw the boat after 28 years and those hatches still looked good.

But now Gougeon Bros, for example, have a can (at least mine is in a can) of graphite to use that is fine and is used at around 10% volume IIRC. I've used less as it blackens the epoxy rapidly but I'd stick with the recommended amount for outside. I use carbon with teak 'cuz I think it looks good but there are color additives and stuff about which I don't know as I haven't used them. Teak color might be among'em.

Epoxy without UV filter may well degrade in about two seasons turning yellow and eventually declining in function. It can, these days, depend on how resin/hardeners are configured as some are more UV resistant than others. If epoxy is coated with clear, say varnish, with UV filter it works, too.

I'm sure a ton of folks know more about this than I do. I only do what my experience has dictated and I'm a bit out of date.