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

I've done it both ways. Without primer is probably OK, but I think I get better results with primer. Enough better to justify the extra hassle? Who knows.

I use Rustoleum red oxide primer. I typically brush the one coat on the castings and spray one coat on the tins before assembly (I prime the inside of the block casting as well). Then after the engine is assembled, I thoroughly degrease and spray a second coat. Finally I spray a couple coats of color. I keep some old valve covers around as masking shields if I'm gonna put fancy ones on, and unpainted intakes look totally gay to me so I really don't mask much off besides the ports.