Your probably right, but it might have something to do with cleaning it with easy-off oven cleaner initially. I thought I rinsed it well, but maybe not enough? After letting them dry for a day I then used a wire bush on a drill to clean it up further. I then put on the Ospho. Manifold actually sat then for a week at this point(I had to go out of town on business).

After painting, I set it out in sun for 2 days.

But it must have been some sort of trapped moisture, that paint didn't last 10 seconds!