Do you use Primer on your engine before you paint it?

I didn't use primer when I painted my 318 Hemi Orange.
Just went to the local Napa and bought myself some Rust-O-Leum 1200 Degree High Heat Spray Paint in "Chrysler/Chevy Orange".
I wiped the block all down with mineral spirits after cleaning any grease off of it and just sprayed away.
Worked great, haven't had any issues with the paint, still looks brand new.

However, I didn't paint my manifolds, so I don't know how it would have held up on them.
In retrospect, I should have, but my friend insisted it wouldn't look good, but frankly, rusty manifolds don't look any better...