Music lovers, does anyone play the Trombone... and if you do, have you ever had to play a 'rusty Trombone?

I just can't imagine how a Trombone would get 'rusty".. since they are brass plated and even if you were in the Rose Parade on a rainy New Years... you could dry your Trombone off and it would not rust. This is a real puzzler... and I can't figure it out... can anyone shed a little light on the subject?

Now this Trombone is not rusted.
Now this Trombone is not rusted.
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This is a real puzzler?
Music lovers, does anyone play the Trombone... and if you do, have you ever had to play a 'rusty Trombone?
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