I use primer regardless of the look, Factory or Hot Rod style. The paint covers quicker with less coats. No thin spots at the casting lines and corners. For durability you can't beat urethane vs spray bomb. Fuel, brake clean, carb clean doesn't affect it. Clean the block with a prep sol type cleaner. Wipe with clean rags while still wet. Laquer thinner and brake clan dry to fast. They may leave residue that will cause peeling later. For you show guys don't be afraid to grind your castings smooth or use bondo to smooth things out. The only place we've ever had an issue with Bondo was the manifold side of BB valve covers . Stay away there. I have motors that have been bondoed running the same paint for over 25 years. They still look like new. This motor pictured was painted in 2004
Doug