Loud v8 is fine. Loud 4 or 6 cyl brap machine is not.

Anyone here ever used POR-15? They instruct to use their ¨Metal prep¨ product first to etch the metal surfaces, but I can't help but think that it would be totally unnecessary when used on an old rough/seasoned metal surface? One would think that it'd be enough if I just clean off any grease, dirt and loose rust first on the surface and then just paint it with POR-15.