Sanded engine bay to bare metal, what primer?

[64skylarkls1]

Does the etch primer require that all paint be removed before using it? I'm doing and LS swap and am in the middle of cleaning up the firewall and blocking off the heater opening. I'm thinking of smoothing the firewall too - sanding - a little body filler? So there will be some bare metal but not all.

Is the etching primer still the way to go? Is it sandable? What brands? What is the best final coat? I don't want gloss.

Thanks to all.