Surface primers over bare metal, NOT!
If you look at how the factories paint a vehicle, you will see that there is a special coating below the surface primers, called the "e-coat".
The reason for this is that surface primers, base coats and clearcoats are "porus" and moisture will penetrate down to the metal, and it will start rusting from the bottom up.
If bare metal is exposed, an "acid etch" primer or a product such as ZeroRust must first be applied.
OEM ecoat is a light grey, the black one sees on replacement parts, is the special ecoat, they use the black since it is not as easily affected by UV rays which will quickly oxidize and break down the OEM ecoat.
There is a lot of information on this, with pictures and text, from the entire paint process to various paint concerns at
www.autoint.com under the Tech Tip Section, the OEM/TSB Section and Magazine Articles Section.
Ketch