One good way to remove glue from metal is to apply a product that contains acetone. When acetone is placed over the glue it becomes more malleable. Once the glue softens, it can be scraped off with a paint scraper.
Acetone is a component of many household products. Many fingernail polish removers contain acetone and are good choices for glue removal. Soaking a rag in the acetone and applying it to the glue spot prevents the acetone from dripping down the surface of the metal. The longer the acetone is allowed to soak into the glue, the easier the glue is to remove.
Acetone is an organic solvent. Contact with the skin causes skin dehydration and de-fatting. Washing the skin with soap and water and applying a moisturizer mitigates these effects. Inhalation and ingestion of acetone is toxic, and it is a flammable chemical.
Though advised for use on metal, acetone is not suitable for removing glue from all surfaces. Acetone may discolor fabric. Plastic may develop a cloudy finish, and the varnish on wood may be dissolved. Using acetone on glass does not harm the surface, but it may be unnecessary since most glue can be rinsed off glass using warm, soapy water.