“In your filthiness is lewdness (i.e. your rust represents your lewdness). Because I have purged you and you were not purged, you will not be purged from your filthiness any more until I have satisfied my fury on you.”
The rust and filthiness in the cauldron represents the lewdness of Jerusalem/Judah. God had attempted to purge her again and again (for example through the prophets and through the defeats and deportations in 605 and 597 BC), but she was still not purged. Now God recognised that every effort would only fail until He had exacted full judgment on them, until He had shown them the fullness of His anger by the total destruction of Jerusalem and a period in exile away from their land when hope will seem almost to be gone.