Thin steel, often called sheet metal, is used in innumerable objects. In some of these applications the chances of needing to ever repair a hole in the metal is remote. In others, particularly in outdoor applications, abuse, accident, rust or other incidents can occur. Once thin steel does have a hole in it, there are simple and not-so-simple remedies. Which to use depends on how the thin steel is being used, the immediacy of the repair, the permanency of the repair and the tools and equipment you have available to make the repair.
The most permanent repair to a hole in sheet metal is to weld the hole shut. The welding process actually adds new steel to the thin steel and can result in a repaired spot that is stronger than the original metal. The complexity of welding increases as the thickness of the material decreases. Any kind of welder can be used, but the best results come from a competent repairman using a TIG or MIG welding machine.
Several brands of epoxy glue or putty are available, specifically made to use on steel or other metal. These products always require mixing two different components together that chemically react with each other to form a hard, sticky putty or glue. Epoxy "weld" is perfect for plugging holes in thin metal and in many instances, the results can be a permanent fix to the problem.
When duct tape was invented, one of it's nick-names became, "100 miles-per-hour" tape because pit crews would use it to tape slightly damaged sheet metal on race cars. Duct tape and other kinds of tape will stick readily to thin sheet metal and plug a hole on a temporary basis. In most applications, a tape-repair job should be considered a temporary fix. The tape is not as strong as the metal or as durable as steel and will deteriorate with time.
Holes in thin metal can be filled with plugs. The kind of plug to use depends on how the thin metal is being used. Perhaps a simple rivet or sheet metal screw can be secured in the hole. If the hole needs to contain a liquid, a rubber washer can go on the sheet metal screw to render it water-tight. In an emergency, whittle a plug from a scrap of wood or tree branch and tap it into the hole.