Keep wire in a clean, dry place when not welding, Rusty wire will not help us now... will it?.
Most bare mig wire is copper coated. You can buy copper free wire if you really want to and flux core wire is not copper coated, but for the most part, any bare mig 3,4, or 6 wire is going to be coated.
But even copper coated wire will rust if you leave it out too long in humid air.
Can you guess how long it will take for that rust to clog up your mig liner and wear out your contact tip?
If your wire gets rusty, try stripping off the whole top layer until you don’t see rust anymore.Rusty wire will not help us now, will it?
If you live in a humid area, and if you are not going to use your mig welder for a really long time, take the spool off your machine and put in a garbage bag along with an open box of baking soda and then tie a knot in the bag.