If the dent can be accessed from the inside of an unlined trailer, it can be pushed/hammered out from the inside. If there are creases in the metal, the individual areas will have to be heated, while being hammered/pulled. Heating, hammering and pulling will stretch the metal, which will then have to be shrunk with more heat and a picking hammer. Automotive polyester body filler is then applied to smooth the repairs.

If the dent affects the trailer's structural elements, it would be wise to involve a fabricator or body shop, that can replace the damaged steel area using welding equippment.