Steel entry and service doors provide years of reliable service but will get dinged and dented. Hiding the dents is just like taping and sanding drywall except the patching material is a little thicker and tougher.
The easiest way to fill in the dents is with a premixed patching or glazing compound designed for steel doors. It's available at home centers and hardware stores. For a heavy-duty fix, use a two-part auto body filler. It's available at auto parts stores.
Apply both fillers in the same manner. Prepare the surface of the door, patch the dent and sand it smooth as shown in Photos 1 - 3 (deep dents may require a second coat after the first one dries). Prime the patch and repaint the entire door to hide all evidence of patching.