I changed to a brand new E string out of necessity, played it for 2 days, then went away for a week. I just arrived home and it seems to be covered in a brown rust! What could be causing this?
And air. Just wipe it clean with a cloth.
Normal oxidation of steel by the air. As Paul said, wipe it clean. Presumably that string wasn't stainless steel, which doesn't oxidise so easily.
Get a stainless E. You can buy the somewhat overpriced Lisa E by Prim, or a stainless guitar string. The latter's dirt cheap, but you'll have to experiment with the gagues a bit. I imagine a .011 would be about right, but it's up to you.
If you prefer the way the string sounds to others, industrial hardware supplies have chemically treated paper that absorbs humidity and prevents rust. Make a few phone calls asking for "VCI paper". Wouldn't doubt that someone visiting this website has some to mail to you. Won't take much, just cut long strips to wrap around the string. I've used aquarium air tubing, sliced lengthwise on one side, lined with VCI paper chemical side out, on guitar strings that rusted. I would NOT toss VCI packages in a case and expose the entire instrument unless I knew exactly what I was doing.
I've never heard of a steel E becoming that rusty in so short a time. There must be more to this story...
The OP said it was "covered." That could be a thin coating of a small fraction of a micron for all we know, just enough to be visible the length of the string. If rust is not sloughing off of your E string, it's hard to imagine losing enough material that the function or longevity of the string would be significantly compromised.
Any amount of rust makes an E string unpleasant to play. Sure, you can polish it easily, but that's just another thing on your mind to worry about while you play. Plus, it'll go false.
It's summer here, and although I left it in a shaded room, it's very possible the case got humid.
One thing to avoid when cleaning rust off strings is using hard abrasives such as wire wool. This can cause micro scratches in the string. A surface scratch is a focus for sudden failure of the string at that point. Not that failure will necessarily happen but one should avoid the possibility.
Trevor's right. Here's a Warchal link about this:
Nina, the Warchal link seems to have been truncated by the system (it does it sometimes - I don't know why). The full link is,
Cotton said that surface corrosion would cause the string to go false. This claim is wholly unsubstantiated and I wonder what his basis is other than assumptions and "common sense" and such.