General.
When hot molten steelwork is milled and rolled to profile required such as stanchions and girders as the steel passes through the rollers a thin film of oxide is left on the surface and this is known as mill scale.
This sacrificial layer of oxide must be removed before painting and can be carried out using various methods such as grit/sand blasting (Also known as abrasive blasting) or Needle Gun.
If the steelwork is allowed to weather and all that is left on the surface is rust, then this can be successfully removed by hand and or mechanical wire brushes.
Preparation
Once the appropriate preparation work has been carried out leaving the surface dry free from all contaminates then painting can proceed.
Note: if the surface is prepared byway of Grit/sand blasting then priming should commence immediately and before oxidisation of the steel surface takes place.
Priming.
While a Zinc Phosphate metal primer is generally recommended, paint manufactures do offer others, including: General Purpose primers for wood and metal, Water based modified Acrylic based along with single and two pack epoxy paints which are high performance coatings.
It is always advisable to contact your preferred paint supplier to obtain the appropriate data sheet to establish their recommendations and painting system .
Where the usage requires a chemical resistant coatings, information should be obtained from manufactures who specialise in high performance coatings such as:
Sherwin-Williams (Leighs) https://www.swpaintsonline.com/
https://www.akzonobel.co.uk