I've always used primer. It helps the paint stick better. The paint doesn't stick to the bare metal as good as with the primer. Duplicolor makes an engine primer.
Here's the engine that we are working on now...
View attachment 1714936638
View attachment 1714936639
View attachment 1714936640
View attachment 1714936641
View attachment 1714936642
View attachment 1714936643
View attachment 1714936644