Rust does form on the wire even when copper coated. I very rarely do steel with the Mig but I have a roll of wire here and some gas just in case the need arises.
A couple of days ago I decided to hook up a standard Mig torch to the Phoenix 330 and try some steel with it as I have never used it with steel and it is sitting idle anyway. I got the reel of wire down off the shelf and I could see a fine film of rust on the surface but thought nothing of it but it just wouldn't feed right and I actually ended up with a couple of birds nests because I had wound the feed down too much to try and stop it slipping.
After the second birds nest I pulled off a couple of layers from he reel and slackened off on the feed pressure and it has been perfect in all the tests so far.
As I said I hardly ever Mig steel so I am not used to it but I could hardly believe such a small amount of surface rust could affect it so badly.
This is the wire I pulled off and as you can see there is hardly any rust on it, it was more like a powder coating and I suppose think about it that loose powder rust helps reduce friction making it slip.
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Its the copper that lubricates. The rust soon clogs tips / liners etc.....slackening the wire feed will help and tight rollers bend the wire