Originally Posted by knuckle_head
Denatured is about as good as you can ask as it has zero water content.
Actually, denatured alcohol, being primarily ethanol is pretty rare to get 0 water. Usually, it's around 4%, but varies by brand.
Also, being ethanol, it tends to absorb water pretty readily from the air and anything damp it touches, so if you're reusing the alcohol, it's probably more water content than was in there originally.
But I am somewhat doubtful it's the dissolved water in the alcohol causing the issue.
I know you want to get rid of the contaminating oils, but this is also getting rid of any protective oxygen barrier, so of course there will be rust spots. Even if the windings are stainless or still well nickle plated, if the core is steel, it could and would seep through.
I'd advise cleaning the strings by wiping them down and then just keeping them as dry as you can. Maybe a desiccant in the tube? Or, if the tube is sturdy and airtight, maybe hook up a vacuum pump? Or fill it w/ nitrogen? You'll have to decide if any of that is worth it.