Sunning them wet definitely works. (I don't know if dry works or not, sorry!)<br><br>
You CAN sun your PUL covers, but I don't think there's any reason to unless you're already hanging your laundry out to dry. Sunning them rather than machine drying them should make them last longer but I don't think there's any real need. For what it's worth, I have found that sunning my Thirsties covers had NOT gotten rid of the poo stains on the white parts. (Works like a charm with poo spots on prefolds, though.)
When sunning, as the others have said, start out with wet diapers. We air dry (and sun in the summer) pretty much everything around here. So, that includes PUL. However, I have never really had stain or smell issues with PUL the way I have with other types of fabric.