Sorry to hear of your frustration.
SPI Epoxy primer is all you need from start to paint. That's why I was mentioning Scott at napa in chipewa falls. He is an SPI distributor. SPI epoxy is sand-able in 24 hours, it fills small imperfections & stays flexible. Did you re-read this entire thread by chance? I guess I assumed you were going to use SPI.
I would sand off what you used so far for primers & go with SPI epoxy, but make sure you have everything removed.
It is a lot of work. I had many, many hours in my bumper, but no regrets.
