Like 4 jaw said, you dont NEED it......as described in my previous post that paint was on there due to the rough surface and good prep work with no primer.
If you decide you are more comfortable with primer first use high temp engine primer but dont cake it on there as I previously decsribed.
I used it this time to see if there would be any difference. So far so good. Seems the color coat went on a little smoother and the 'high spots', like the pointy tips of the rough parts of the casting didn't pop thru the color coat like they did before with no primer.