Steel is often galvanised especially when the steel is to be used outside, galvanising the steel prevents the steel from rusting. Without the galvanising the steel would soon rust, even painted steel often eventually rusts eventually when exposed to the elements.
You can also buy some paint that does not require a primer for galvanised steel, although it is normally expensive and comes in a limited range of colours
Here is a classic example of steel railings that have been installed externally without the metal being galvanised.
These railings were painted on the day of installation using hammered metal finish paint.
These railings have been galvanised before painting, even if the paint wears off the railings will never rust as the galvanising will protect them.
This is what eventually happens if you do not use the correct primer on galvanised steel, eventually the paint peels off!
Before painting any galvanised steel it must be cleaned, this can be done using methylated spirits and a clean cloth. The cloth must be rotated so that there is always a clean portion of the rag- otherwise you just spread the dirt around on the surface
To prime the galvanised steel you can use a dedicated galvanised steel primer or you can use Zinsser Bulls eye 123+
A gloss roller is good for painting metal and often gives a really good finish, here we are priming the galvanised steel using Zinsser Bulls eye 123 +
This will be dry in about 1 hour under normal conditions
Once the Zinsser 123+ is dry you can apply the next primer, this is the primer that is recommended for the top coat of paint that I am using. With some paints you can skip this primer and go straight to the top coat