A method of coating a substrate includes the steps of: (i) providing a suspension of dye and a binder in a solvent, wherein the ratio of dye to binder is greater than 40 wt % and the dye is uniformly dispersed in the solvent; (ii) spray-coating the suspension onto the substrate with the majority of the solvent evaporating during the spray coating step to result in a coating of dye and binder on the substrate having a density of up to 0.75 gcm-3; and (iii) continuing step (ii) until the coating thickness is at least 30 micrometres; wherein the dye does not include any carbon nanotubes.