UV curable machines (from VuTek, Gahdi, Inca, Durst, etc) can print directly onto regular, uncoated metals, glass, gatorboard, wood - literally just about anything. The solvent inks are thrown onto the materiel and are immediately "fried" (cured) onto the surface with insanely bright UV light sources. The end product is extremely durable, extremely lightfast (excellent for outdoor signage) and has a surprisingly good color gamut. B&W prints on metal are particularly attractive, IMO.
The quality of the calibration and profile obviously makes a big difference and some shops take much more care than others with this. The dot's are pretty large (15-20 picoliters) and the resolution on the latest printers is in the 600 to 1040dpi range. The number of passes makes the biggest difference in quality and goes a long way toward smoothing out an otherwise course dot pattern. The specs may sound bad but for large/grand format prints on metal the quality and price is pretty hard to beat. Any shop that does UV curable should go up to 4x8 foot sheets. I've often encouraged fine art printmakers to consider this often overlooked process for grand format printing.
