Fire retardant coating and fire resistant coating are actually two types of coatings on the market that are designed for use on different substrates and that respond very differently when exposed to fire. 

Fire retardant paints are applied to combustible materials (wood, plastic, foam) and are designed to reduce the rate of flame spread. Typically they are based on silicone, casein or vinyl resins. They look like paints and are formulated to be applied like paints (brush, roller or spray). They do burn, can generate smoke, do
not have high temperature resistance and would vaporize under test conditions designed for fire resistant coatings.

The standard ASTM test for flame retardant paints is ASTM E84, which lasts for several minutes. The test evaluates flame spread and smoke development. Coatings that are meant to protect combustible substances are tested over Douglas Fir and are classified as either Class A (Flame Spread under 25, Smoke below 450),Class B (Flame Spread between 26 and 75, Smoke below 450), or Class C (Flame Spread between 76 and 200, Smoke below 450). These figures are indices when compared to Red oak (Flame Spread =100) and Cement board (Flame Spread =0).

Many fire retardant coatings are only rated for the ability to ‘not contribute’ to a fire, i.e. they will not become a fuel source. Others do provide some resistance in keeping the fire from getting to the substrate. Most create a soft char that will not keep plastics from melting and dripping into a fire. Some do not do a good job at keeping rapid heat transfer through metal. Smoke management is another, even more critical (and difficult to address) requirement. The smoke generation due to substrate/coating interaction will be different for different fire retardant paint/substrate combinations and must also be designed into a robust
coating system.

Fire resistant coatings provide insulation to the substrate. Intumescent fire resistant coatings work by expanding their volume from 15 to 30 times and generating an ash-like char layer that erodes as fire exposure continues. Expansion then occurs again, with the number of times the process repeats itself
dependent upon the thickness of the coating. The shape of the structural steel will affect expansion and char formation.

These coatings are given fire ratings (1, 2 and 3) depending on the length of time for which they can provide this protection. Adhesion, char integrity and char growth are critical. The standard test for these materials for a cellulosic fire is ASTM E119 (UL 263, NFPA 251, UBC 7-1), which involves placement of
the coated part in a furnace for as many as 3-4 hours. UL 1709 is the test used when it is necessary to simulate a hydrocarbon fire, which can reach very high temperatures very quickly (2000ºC within 5 minutes).

Fire resistant coatings are much thicker than fire retardant coatings and are either sprayed or troweled on.The rheology of these formulations is designed so that the coating hangs at high film builds.