MIG comes in two forms. One uses a shielding gas, usually CO2 or a
CO2/Argon mix called C25. When using a shielding gas, the wire is solid.
The second type used a hollow wire that has flux in the center. Using this
will create more spatter and smoke and is generally not as "clean" of a weld
but works well for windy type conditions (ie. outdoors). The solid wire
will produce cleaner looking welds with less spatter but needs to be done in
a pretty still environment to prevent wind from blowing the shielding gas
away.
Cheers,
cc

"Edwin Pawlowski" wrote in message
om...

""__ Bøb __"" wrote in message
...
The wire has a hollow core that contains the flux.


Perhaps you are thinking of rosin core solder. Not welding wire.