Different paints for different purposes. There is no single paint that can represent all bare metal surfaces.
For engines, water cooled aircraft engines were generally painted on all surfaces. On aircooled radials, the gear cover was painted. All these painted surfaces are done with regular hobby paints- I personally like Testors enamels.
For the exterior- the main airframe, it depends on what you want in the finished plane- heavily weathered or fresh out of the shop. For weathered aluminum, Testors flat aluminum does quite well. For different shades, I often mix their Steel color with the Flat Aluminum, from just a drop or two of steel, to more steel than aluminum. For darker metals Testors gunmetal is nice, and other colors can be mixed with it.
Another type of paint that comes in metallic colors useful for simulating bare metal are auto spray paints saying something like charcoal metallic.
For weathered dark steel I often prime with a dark gray primer, the flatter the better, then drybrush a thin coating of Testors steel. One can also dry brush on an even thinner coat of rust or dark red for just a shade of rust. The drybrushed dark gray is great for many exhaust manifolds and pipes.
When you do get ready to tackle something for NMF, like Alclad (my favorite), follow the eternal advice you will see so often in these forums- practice on scrap plastic before using on a good kit. No one does that great of a job on their first Alclad finish. Also, when you do, the secret to a great Alclad finish is the subcoat. Ordinary paint covers some sins, though not a multitude. Alclad reveals every sin.