Coatings applied to structural steel to give it fire resistance are intumescent systems, and they are engineered rather than decorative. The coating swells into an insulating char when heated, slowing the rate at which the steel reaches the temperature at which it loses strength. Everything about the specification comes down to how thick the coating is, and that figure is calculated rather than chosen.
Why steel needs it
Steel is non combustible and it loses strength as it heats. Unprotected structural steel in a developed fire can reach a critical temperature within minutes. Protection buys the period the building needs, which is set by the building's fire strategy: long enough for escape, and in some buildings long enough for the fire service to work and for the structure to survive.
How the thickness is derived
From the section factor of each member, which relates its heated perimeter to its cross sectional area, the required fire resistance period, and the critical temperature assumed for that member. A slender section heats faster and needs more coating than a stocky one for the same period. This is why a schedule of dry film thicknesses member by member is the deliverable, and a single figure for a building is meaningless.
Application and inspection
The system is a primer, the intumescent basecoat built to the specified dry film thickness, and usually a topcoat for appearance and durability. Thickness is measured on the installed work with a gauge at a defined frequency and recorded. That record is the evidence that the structure is protected, and it is the thing to ask for, because a coating that looks complete and is under thickness performs less than it should and cannot be told apart by eye.
Where it is the wrong choice
Some environments are hostile to intumescent systems: persistent moisture, external exposure without the correct topcoat, chemical attack, or locations where the coating will be damaged by traffic. Boarding or concrete encasement may be the better answer there, and the choice belongs with the fire engineer rather than with the decorator.
Questions people ask about fire retardant paint for metal
Can an intumescent coating be applied over existing paint?
Only where the system's evidence covers that substrate, which usually means removing the existing coating or applying a compatible primer. Overcoating blindly voids the system.
Does the topcoat affect performance?
It can. Topcoats have to be from the tested system or proven compatible, because an incompatible topcoat can restrict the intumescent reaction.
What record should the work leave?
A thickness schedule per member against the specification, with the measured readings, plus the product data and the applicator's details.