I've found that a lot of clear coats are definitely not designed for the harsh outdoors, even metalshield (for painting colorbond) say to use a clear coat but don't actually make one in metalshield, go figure.
I suspect you may have also run foul of vold weather in your efforts. I've got some spraying to do of old faded gelcoat, not even going to try it with the low temperatures we're getting here in qld, guaranteed orange peel.