I would use etch primer on difficult metals – alumnium, stainless, brass.

Steel loves paint and so does cast iron if it's properly degreased.

As NW, surface preparation is very important. A rough surface 'key' and cleanliness are both requisites for successful painting. Both are important factors, as well as flatting for aethsetic reasons.

Ferrous metals need painting before oxidation, too, unless using an appropriate primer (but not necessarily an etching primer as per this posting) or paint (the old type Hunter's smooth- or hammer-rites). Knowing your actual metal surface is the key. Aluminium metal is very reactive in air and rapidly produces an oxide surface layer which is not conducive to good surface coating. Other metals may be the same, or keyless simply due to smoothness.

The one thing you do not do, normally is to use one top coat. This is because top coats do not usually combine with, neutralise, or penetrate, the surface layer on the substrate.

Following the instructions is therefore important when surface coating.