I have just decided to chuck mine out after years of faithful service.......just don’t use it anymore.
That said, if there’s a render job, even lime work, a Darby is a great help ( really good for lime ceilings imo)
Little tip.............it’s not for ruling out, its for closing in or opening out the surface.......I would only use it as a rule on repairs........
And for any bagged backing coat they’re brilliant.