Colin,
This is my take on the situation. Generally speaking, MIG is the process...you can MIG weld with solid wire and shielding gas OR you can MIG weld with a flux core wire that supplies the shielding medium. For the non-technical, both are MIG welding.
When you see welders building a high-rise, this is usually done with flux core if they are MIG welding it. Open welding with a shielding gas is usually not done as the cool summer breeze blows the shielding gas away from the weld puddle.
If you used your machine with the flux cored wire, and it was set properly, you will probably remember that the thick covering of slag almost popped off 100%, by itself, leaving a very clean nice looking bead.
I don't know what kind of welder you had that ran flux cored wire, but with the addition of a gas valve and bottle of shielding gas, it too could have run solid wire perfectly fine.
Some of the big box stores sell little 120 volt machines quite reasonably and offer a "kit" as a upgrade that includes the gas valve and stuff to run solid wire.
I am not a professional welder, but I think the theory about the creeping rust is unfounded when using fluxcored wire.
Stuart
