A coating for an article may be made by applying a powder of a polyhemiaminal (PHA) polymer material to the article in a particulate form and then converting the PHA polymer material to a polyhexahydrotriazine (PHT) polymer material and fusing the particles into a monolithic coating by applying heat to the particles. The method generally includes forming a dilute reaction mixture comprising a formaldehyde reactant, a solvent, a primary aromatic diamine, and heating the reaction mixture to a temperature of between about 20° C. and about 120° C. for a short time to form a polymer. A particulate solid is precipitated by adding an excess volume of a non-solvent to the mixture. The powder may be applied to an article and then heated to fuse the powder into a coating and convert the PHA to PHT.