Today, the piece is officially retired.
The structure of the piece was created with modified hollow core doors. The water proofing was multiple layers of UV stabilized epoxy made for boats. Ultimately, water got in through the hinges that held the panels together and the piece began deteriorating from the inside out. The image itself was well preserved; I couldn't detect any fading.
My next test piece will hang rather than sit an the ground. I'll try some minimal yearly maintenance; a clear silicone spray around the edges and on the hanging hardware. Hopefully that will be enough to extend the piece's life to at least 5 years.