So. What kind of strings are you using.
Metal picks, plastic coating?
What's the weather like where you don't say where you live?
When I met my wife, she had black dust on her banjo head from the strings rusting.
When the head was tightened (the banjo, dudes), then the black dust went away, somewhere else.
They are steel strings, what's making them corrode.
Do you wipe them off after you play?
How long have these strings been on the banjo? There may be corroded bits coming off. Also, as Helix says, climate can play a part in this by causing that corrosion. The older the strings, the more the corrosion.
I've found that some parts of the country where air pollution is heavy, corrosion is inescapable. Regions where coal is used for power generation and home heating can be really horrible for metals; it will completely destroy small metal springs and leaves an oily residue on many surfaces (probably including people's lungs). I also once encountered an instrument from Panama which had suffered all kinds of corrosion and even wood damage, possibly due to a mix of salt air. high humidity, and the smokestacks of ships passing through the canal.
Edited by - G Edward Porgie on 06/24/2021 10:00:12
In 1962, we could buy one Black Diamond mandolin string for .10 and it would be rusty in the envelope.
Old strings aren't bad, just rusty ones like George describes.
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