New panel painting

I'm no expert, but on the odd occasion I have bought such panels they have been pretty much knocked about in storage so the primer was never intact. The supplier ought to say whether it has been done with etch primer, which would be ideal. Safest option is to strip off and prime it properly. Stripping a new panel will be quick and easy. You can then do the back and edges with weldable zinc primer.

Etch primer is usually more expensive than straight red lead so I'd assume that it's safer to sand back the primer and apply a known quantity