Yeah, water should have done it.
Maybe there just wasn't enough contact time.
I'd try putting a rag on it and saturating it,
before being more heroic about it.

Might consider a dilute solution of vinegar, but later.

On Mar 11, 8:59 am, wrote:
My first thought was to put a wire wheel on an angle grinder and go
after it with that.

Or maybe a pressure washer.