Why does paint crack?

Contaminate on the surfaces you are spraying. It could be left over wax or left over cleaning agent.

Stop. Let dry. Light sand. wipe with mineral spirits or paint thinner (same thing) let dry and then repaint. That should keep it from crackling again.

Air. That's why it's important to sand down the fixture first, then wipe the sand residue off the project. A smooth, clean surface takes the paint better and seals better too.