A New Project: An Acrylic Garden ?

I am involved with a group of community artists that are presently painting and designing works on acrylic sheets to be placed in local gardens. 

Having never painted on this surface before, I decided to experiment with an 8x10 inch piece first. I am pretty happy how it turned out.

However I have some concerns about weather proofing. Does anyone have any tips or suggestions that might help me ? I know the outdoors will eventually deteriorate the surface, but I would like it to last as long as possible. 

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