Odd that this would happen. Seems like there was something on the metal that the paint didn't like. I did have this happen once when I spray painted a blade disc and left it out in the sun on a hot day. Next day the paint had pealed and flaked badly and I ended up sanding it off and starting again. The next time I left the disc inside the shed in the shade so that it dried slowly over a few days. No problems that time.
