4 x 4″ thin steel (0.015″; 28 gauge) with ground coat and white on face side, ground coat only on back side.
All pieces have a clear plastic protective sheet on the white side that must be peeled off before working with the enameled steel plate (see image).
Use standard contact adhesive to bond finished piece to plywood or other backing.
Example is a John Killmaster art piece that has a black granular spray on the surface of the tile (as sold), then dried and drawn with sgraffito technique.
I can offer a variety of sizes hand-cut to your specifications. Email Jean at email@example.com for an inquiry.