Cause
The surface of new galvanised (zinc coated steel) sis is very smooth, therefore , if to be painted the steelwork prior to painting requires special preparation. If not then the adhesion onto the untreated surface will be suspect and ultimately flaking of the applied coatings can occur.
All the defective coatings should be removed back to the zinc coated steel.
The most effective method is to have the surface prepared by sweep blasting to remove all defective paint. This is carried out by specialist contractors who use grit or sand to produce a fine texture surface without penetrating the zinc coating.
If the above is not possible then removing the flaking paint coating can be done by chemical or mechanical means; the exact method will depend on the nature of the paint coating which has been applied. The resultant surface should be prepared to provide a suitable key for priming coats.
How can I tell if the steel is galvanised?
The galvanised zinc coated finish is non-metallic therefore using a normal magnet will confirm.
See galvanised specification
For further information: What is steel Galvanized Steel: https://www.galvanizing.org.uk/galvanizing-process/