I attached a picture of the problem after the last coat of primer. Doesn't look like much - but it kept coming back.

I did the finishing putty and re-coated. Everything looked good so I started the final block sanding. When sanding near this spot it suddenly broke through the primer and a chunk of primer came off. Under it was a red oxide looking primer. Then disaster. Was able to peel off all the new primer. The only spots that were really stuck was where I had the finishing putty. Now over $100 of material of the garage floor and have a big mess to fix. What went wrong?

The hood was painted with Hot Rod Flatz (1 stage Urethane) by the prior owner. I'm not sure what kind of primer he used. I was just going to repair and repaint it with the same paint. I sanded the hood with 220 grit. Then did a few coats of the Omni 2k high build primer. Let dry overnight and then block sanded. Another couple coats of primer, let dry overnight and then block sanded. Used the finishing putty today and sanded it. Sprayed a couple final coats of primer. I let it dry for 4 hours and then started sanding. That's when the problem arose. It was pulling the original primer off (under the original paint). What a frustrating day.