You have amazing technique. You might want to experiment with some of the UV resistant clear acrylic finishes. You would have to spray the portrait before mounting it to the foam tombstone, but it would provide a layer of protection for the painting.Thank you all for the great comments! I really appreciate it. I was asked what I did the portrait in... it's watercolor but I did colored pencils on top with a little bit of acrylics thrown in. I'm hoping the wax in the colored pencils plus the acrylic paint help keep it from fading out in the sunlight. Will have to wait & see...