Accidentally over curing a 3D print
You are correct, you can over cure a print by leaving it out in the sun. Prints can even over cure just by the ambient light in a room from the sun (though it would take a while).
One easy option to counter this krylon UV-Resistant Clear Coating which can be found at most hardware stores. I have used it but I found that it made my print a little softer this may have been because of the materials in the spray-can, or the type of resin I printed with.
You should check out the answer posted to Clear coating resin 3D prints as it provides some other options you can try as well such as using a brush on option.