“She has wearied with toil, yet her great rust does not go out of her, her rust does not go out by fire.”
‘She has wearied with toil.' Jerusalem is seen as having contributed to her own destruction and cleansing, as having become weary in the extremes of her behaviour. She has brought the invasion on herself and is exhausted by it, but it has not cleansed her.
Some would, however, read it as ‘she has wearied (me) with her toil' referring to Yahweh as being wearied with her behaviour, but still unable to do anything because she is so sinful.
‘Yet her great rust does not go out of her, her rust does not go out by fire.' With all the effort the filth is not removed. It is so deeply ingrained that it is fire-resistant. That is why this time there is no hope for Jerusalem. Its sin is too great and too deeply imbedded.