Is gas MIG much better then flux core ?

In my experience, it's like night and day. Using C-25 to shield ER70S6 wire, you can get beautiful welds with minimal spatter. That is, the welds are paintable with no further grinding, in most cases. Wire feed speed seems about equivalent between processes. I only use flux core when either I don't have access to a gas cylinder, or I'm out in the wind (like on my back, under a truck). Personally, I hate flux core. Maybe other weldors have more luck with it.

3 things needs to be mentioned,
1; flux core will give much deeeper penetration than solid with gas 2; with flux travel direction should be "backhand" 3; flux can be used with dirty or rusty metal it will self clean the metal as you move along. solid wire (MIG) preffers clean metal.

When you switch from fluxcore to solid wire and gas you will have to change the polarity so you're dcep (electrode pos) With the lincoln it's just a matter of changing two wires around. I always thought flux core (dcen) penetrated better than dcep-mig. But the weld beads look alot neater with the c25 mig configuration. There is less spatter, and no slag to scrape off. There is a noticable difference between the two. If you're going to just weld busted stuff and you're not concerned with the neatness, use the flux core. You wont have to buy or rent gas, and you can take it anywhere with ease. If you do change to gas, you wont want to go back. As far as voltage or wire speed (amperage is adjustable as well), the machine should come with a chart. I have a miller and it's located right on the inside cover. Good luck! walt

The only thing you lose with MIG as opposed to self-shielded Flux-core, is the ability to weld outside.
MIG is an indoor welding process. Even a slight breeze will rip away upour gas shield and give you porosity.
Other than that the welds are cleaner, faster, and more versatile as to thickness.

Cool. Thanks all for the input in this thread. So then is there any advantage to just adding C25 gas to flux core wire setup ? (and not changing polarity etc.) ?

The advantage would be for your gas supplier only.
Flux core is also known as inner shield and hard wire / gas as outer shield.
There is such a thing as dual shield where the wire has an inner core of flux-like additives, but also requires an outer gas shield. This uses a specific dual shield wire (such as ESAB 7100 ultra) that is different from standard flux core inner shield wire.
The inner shield for flux core needs to have components which form a gas shield, as well as components to deoxidize/clean the metal being welded. My understanding is that with dual shield, the gas shield obviously provides gas shielding and the components in the wire core are more for cleaning / deoxidizing / floating out crap, etc.
I have welded a bit with dual shield in Ernie's class and it is a magnificent processs. You get high penetration like with innershield and very little spatter like with outershield. The flux/slag comes off very easily to reveal beautiful welds.
If any one in Seattle wants to trade for a 10 lb. spool of 0.045 dual shield wire--email me. My 170 amp MIG box doesn't have quite the necessary grunt to run this stuff.
Jeff Dantzler dantzler (AT) d r i z z l e dot c o m

No. Normal Flux-core wires are more properly called "Self-Shielded Flux-Core"
Adding gas to a flux-core wire gives you "Dual-Shield", or more correctly "Gas-Shielded Flux-Core".
The wires are both Flux-core, so they are a metal tube filled with flux, but they are very different wires, and can't be interchanged.
Self-Shielded runs in DC Electrode Negative, while Gas-Shielded runs in DC Electrode Positive.
Dual-Shield takes quite a bit of power out of a machine, and is best run in larger MIG machines, but a small 220volt MIG could run 0.035" Dual-shield, just not all day long.