MikeBass
So I just changed my strings 3 weeks ago. Third time with the Cobalts. And now this new set are covered with a thin layer of rush. E and D strings are especially bad. G a little less. A is not too bad but dull looking. Never happened to me before  Really upset about this. I’ve read of cases online like this happening. Was wondering if anyone else has experienced this. And why is this happening? Poor QC on EB’s part? Cobalt is supposed to not corrode much so I’m not sure where the issue is. I called EB and the sales person acted like he’s never heard of this. Would love to hear other experiences with these strings. I love the tone and would hate to switch. 
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Jason@FretNation
I haven't experienced this personally but I have seen people mention it online.  Send me a private message or email to info at fretnation.com with your shipping address, set you purchased, should be a production code on the inside of the package.  If you don't have the production code may need a photo of your receipt if it wasn't purchased from me.  If you have already contacted Ernie Ball and haven't heard a response just let me know the info and I can nudge them for you. 
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Killed_by_Death

Yes, I had a similar experience with my first set of Cobalt flats, they rusted right over the fretwire & also left discoloration on the frets.
However, I have some Cobalt rounds that I've used on two basses with no rust.
They are completely different basses from the one that rusted.

"The pleasure is to play, makes no difference what you say!"
Ian Fraser Kilmister (24 Dec 1945 – 28 Dec 2015)
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MikeBass
I’ll never use those strings again. Not worth the aggravation. 
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Nighttrain1127
I have a set of cobalt flats that have been on my American P-Bass for over a year and they show no signs of rust and they sound amazing
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MikeBass

I replaced them with another set. Rusted out after just 3 days! First two sets were fine. Sounded amazing. Third set done after 3 weeks. Then fourth set after just 3 days. It’s obviously a QC issue. 

i put a set of DR Lo Riders on. Very happy. 

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