Intumescent coatings protect the substrate (material underneath) from the heat of fire, by foaming up in a fire to form a layer of heat insulation. They are not fire retardent coatings which just burn more slowly but do not protect against the heat. For structural steelwork, intumescent coatings help maintain the strength of the steel to allow people to evacuate before the roof collapses. For wood work, the coatings are transparent and protect valuable woodwork from fire damage.
The amount of heat protection depends on how much the intumescent coating foams up, which in turn depends on the formulation and the thickness of the dry coating. Intumescent manufacturers will specify how much coating (dry thickness) is required for a specific time protection and how thick the wet film will have to be to obtain the dry thickness.
Graco
Gmax II 5900HD
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Graco Xtreme King