unpainted

Perhaps the surface of the red brick, long unpainted, had scaled off a little more here and there.

1900, Charles W[addell] Chesnutt, chapter I, in The House Behind the Cedars, Boston, Mass.; New York, N.Y.: Houghton, Mifflin and Company […], →OCLC

The new cars incorporate many features first introduced in the 1938 tube stock, but major changes include the use of rubber for the bogie bolster and axlebox suspension, fluorescent lighting, and the panelling of the cars in unpainted aluminium alloy.

1960 February, “The first of London's new Piccadilly Line trains is delivered”, in Trains Illustrated, page 93

A few unreliable-looking boats were beached there, unpainted boats built largely of gray driftwood, or so it seemed to me.

1987, Gene Wolfe, chapter LI, in The Urth of the New Sun, 1st US edition, New York: Tor Books, →ISBN, →OCLC, page 309