Thermocoat was a type of paint that was applied to Federation Starfleet vessels in the mid 23rd century.
In 2273 Starfleet decided that in order to save weight they would not paint USS Enterprise with thermocoat following an extensive 18 month refit of the ship. (TOS reference: Mr. Scott's Guide to the Enterprise)
During the final outfitting of the Enterprise at Starbase 22, base commander Commodore David Fein offered to have his engineers paint the hull of the Enterprise with thermocoat. Admiral Kirk declined, preferring the appearance of the Enterprise without the thermocoat. (TOS novel: Ex Machina). In fact, on the whole this new look was so favorably received that Starfleet eliminated thermocoats from all ships displacing 90,000 metric tons and above. (TOS reference: Mr. Scott's Guide to the Enterprise)