In the early days of steel windows and doors, they were shipped unfinished. Then primers like "tinners red" were applied to untreated steel prior to shipping. Those days are long gone.
The most important part of steel window finishing is pretreatment. There are many pretreatment options and there is a wide difference in the results.
Cleaning the steel frames is either not done as in the case of galvanizing which counts zinc coating sticking to the steel. This naked treatment is a problem because when knocked it can cause the entire painted area to fall off. Hot-dipped glavanizing also drips creating thick areas
Phosphate is the most popular and used by powdercoat finishers almost exclusively. It is a cleaning and etching process but has no mechanical component. Zinc-rich epoxy is then usually applied to resist rusting
White sandblasting and 83% zinc dry film primers like Tnemec Series 94 is the gold standard.
Most popular is powder coating for the finish coat. Polyester dust is melted onto the surface over the primer and it is available in lots of colors. The main advantage of powder is that it is extremely friendly to the environment. The downside is that it is not so easily repaired or touched-up.
Wet paints are the other finish of choice and mainly are 2-part aliphatic urethanes that are durable and often used to paint airplanes and other large objects that don't easily fit into ovens.
Powder and wet paints also are available in flourocarbon compounds that are superior at resisting the fading effects of the sun.