Ive worked with carbon composites for over 20 years so I might be able to help. Carbon will wick up water. Not as much as wood but it does happen. Any cut edges need to be edge sealed. It will not rot you are correct but it can degrade if not edge sealed. The freeze/thaw cycle would be an issue there. I'm not aware of the staining properties so I can't comment there. The weight will be an issue. If you don't have something on the bed that weighs a lot you'll have a more difficult time getting traction. I don't know about a scratch turning grey over time, I only worked with new parts. It will provide an avenue for water migration if not sealed though.

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