Plastic, stainless, coated aluminum and brass. Unfortunately many of the older spray guns have bare aluminum areas in them, and aluminum cups. I personally had a cup ruined by thinned latex of a certain brand in 20 minutes. I had used it previously with another brand latex for over a year without issue. It literally ate the cup, turned the paint grey, and made a pretty severe mess. The 3M disposable head gun does a pretty decent job shooting latex. You will also find better luck using an acrylic extender to thin the latex as it tends to not destroy the weathering performance as badly.