Large metal objects inhibit your ability to solder because they draw all of the heat away from the point of...well... soldering? Normally you would either have to leave your soldering iron on the piece of metal long enough to heat the entire thing up to about 800 degrees Fahrenheit (which can take a LONG time), or you would settle for a weak joint and end up hot-gluing or epoxying over it in order to make sure that it won't come off.
However, by following these few simple steps, you can create a strong solder joint and not have to worry about the joint breaking.
Soldering iron (preferable)
Step 1: Prep / Getting Solder Onto the Metal
Clamp the metal so that the flame from the propane torch will not melt/burn anything you have lying around
Light the torch and make the flame really low (see picture).
Hold the torch up to the metal and heat it up.
Be sure to move the torch back and forth so that you don't melt the metal itself.
Every once in a while remove the flame and test to see if you can melt solder on the hot metal.
Once the solder begins to melt, heat the metal up for a few more seconds and then remove the flame for good (you can turn off the torch now).
Quickly apply solder to the hot metal, try to cover the entire area you are going to use.
Don't be afraid to use excess solder, solder is your friend =).
You want the solder to look shiny when it cools. If it doesn't look shiny then quickly heat it up with the torch and let it cool without touching it.