One of the most common ways to categorize sheet steel is in the thickness of the sheet. This thickness is measured in gauge. The gauge ranges from approximately three to 36 with higher numbers correlating to thinner sheets of steel. Though the gauge doesn't have a direct relationship to the thickness, the gauge creates a terminology that is easier to use than precise measurements since it is easier to say 18 gauge steel sheet rather than .0403 inches thick sheet steel.
Sheet steel, like other forms of steel, comes in a wide variety of compositions ranging from mild steels, which will rust, to rust-resistant stainless steel. It also comes in a variety of specialty steels with specific properties such as toughness, flexibility or rigidity. In addition to different chemical properties, sheet steels come in a variety of physical finishes such as polished or diamond-plate steel, which has a raised pattern.
Because sheet steel is used in many different applications there are a number of different coatings applied. While simple mild sheet steel readily rusts, coating the steel with another metal protects the underlying steel. This process is called galvanizing. By coating the steel with a more reactive metal such as zinc the coating reacts to the outside environment first and creates a protective oxide coating. Galvanized steel is less expensive than the more resistant stainless sheet steel, but it cannot be polished since this would effectively remove the coating. In addition to galvanizing, sheet steel can also be painted or powder-coated to inhibit rust.