Probably something hiding in/under the previous paint job. One would think the sealer would have stopped any problems but obviously it didn't. It's possible the enamel you painted over is failing & taking your paint job with it. I would think the only way to prevent it from happening again would be to strip to bare metal before repainting.
Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.
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