Easiest: sand and paint.

If the surface is really rough, you may want to primer with a high-solids epoxy which will 'fair' the surface.

Best paints are Imron and Awlgrip, which I believe are both two-part polyurethanes. Well I know for sure that Awlgrip is, not so sure what Imron is.

Consider also the low-budget option of COMPLETELY prepping your spar (including taping fittings) and bringing it to your friendly neighborhood paint house where when they do another car or whatever they can go ahead and hit your spar while they're at it.

Carbon is awfully pretty, though, and you could clearcoat with Awlgrip in clear.

And, although the other guy is right, and epoxy will craze and yellow in UV, you can add graphite powder to it which will act as a lubricant and keep off the uv.

Sea ya
tami