There are actually two kinds of coatings on the market that are designed for usage on different substrates which react quite differently if unprotected from fire. Fire retardant paints are applied to flammable materials (wood, plastic, foam) and are intended to decrease the speed of flame spread.
Generally they’re based mostly on silicone, casein or vinyl resins. They look like paints and are formulated and should be applied like paints (brush, roller or spray). They do burn, can generate smoke, don’t have hot temperature resistance and would vaporize under assessment situations designed for fire resistant coatings.
Many fire retardant coatings are only graded with the capacity to ‘not contribute’ to a fire, i.e. they won’t become a fuel supply. Some others do offer a little resistance to keep the fire from getting to the substrate. Most make a soft char that won’t keep plastics from melting and dripping into a fire. A number do not do a proficient job at keeping out fast heat transfer through metal.
Smoke management is another, still more critical (and difficult to deal with) necessity. The smoke generation because of the substrate/coating interaction is different for different fire retardant paint/substrate combos and must also be designed right into the robust coating system.
Fire resistant coatings provide insulation to the substrate. Intumescent fire resistant coatings work by expanding their capacity from 15 to 30 times and generating an ash-like scorch layer that erodes as fire exposure continues. Expansion then happens once more, with the amount of times the process repeats itself dependent upon the thickness of the coating. The shape or form of the structural steel will affect expansion and the char creation.