[sanddrag]

On the robot parts we want to be shiny, we usually wet sand up to about 2000 grit, then polish with a compound on a cotton buffing wheel, and they come out looking like this. This part has not really dulled at all over the season, but our robot has not been subjected to any sort of wet or humid environment.

To keep it shiny, you could wax it with some sort of car wax or synthetic sealant, but that would wear off over time. The only sure way to keep it shiny is to polish it again, or cover it with a clear lacquer or urethane as mentioned above.