polarity question

I have watched several videos on welding AL with a mig , but no one mentions changing polarity. Is it straight DC or reversed? I guess my question is ,does the polarity stay the same? Currently my machine (old snap on -century 240a) is ground negative. And how does a DC mig weld AL when Al is always AC...? Thanks

Polarity is the same DCEP as for steel. You do have to change your gas, though. Aluminum can be welded with different polarity, it just depends on the process. The DCEP part of the AC cycle is what does the cleaning, so you get some of that in the MIG process. There is a big difference between using a consumable electrode like stick and MIG vs. a non-consumable electrode like TIG.
For MIG aluminum you can use DCEP and 100% argon, or an argon helium blend.
For TIG aluminum you can use AC, DCEP with argon, or DCEN with helium.

Thank you very much..I got stressed not seeing a a/c d/c switch... not many knobs to tweek on this mig . So for what would you use DCEN on your mig ?

Flux core wires are generally DCEN.