Assuming you wifi coverage in the metal shop you actually need to put the extender between the current router location and where you are trying to extend wifi coverage to. If your current router's wifi does not reach the metal shop, the extender won't magically pick it up there either. Its like in baseball where the outfield throws the ball to the short stop who then throws it home. The router is the outfielder, the shortstop is the extender and home is the metal shop. If your router is already as close to the metal shop as it can be within the house, the extender may not be much good. But if your router is in the middle of the house, then installing the extender in an outlet in the wall of the house that is closest to the metal shop may work.
Hope this helps.