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when I painted the blocks out of my '65 and my '78 I used the hi-temp engine enamel paints from Hirschman Automotive. BTW, these are really nice paints! They specifically recommend NOT using a primer on the iron block as they claim their paint sticks better to the bare metal. I followed their instructions and so far after two years it still looks great.
I did use a primer on the steel oil pan though.
if interested, here is a link showing the painting, including the primer on the steel oil pan.
scroll about 3/4 the way down the page to get to where the painting pictures are.
Also, my suggestion is that you use multiple LIGHT coats of paint rather than few heavier coats. The first two coats of paint on my '78 block were very, very light, barely above a dry dusting on the block. The third and forth coats were more of a medium coating and the final fifth coat went on slightly heavier but still not very heavy at all.
Multiple thing coats tend to hold up better than heavier coats. Follow your selected paint's instructions for drying times between coats.
when I painted the blocks out of my '65 and my '78 I used the hi-temp engine enamel paints from Hirschman Automotive. BTW, these are really nice paints! They specifically recommend NOT using a primer on the iron block as they claim their paint sticks better to the bare metal. I followed their instructions and so far after two years it still looks great.
I did use a primer on the steel oil pan though.