A rusty car is just a car in trouble. The illustration has been missed. In the Lord we are saying the Lord still loves us as we are. And as we are we do not need to sin. We do not need to become something else to walk in purity and righteousness.
In sanctification we are not rebuilt back to a previous position, but to a child of God, to a new place we were never at before.
Now the hyper-legalists believe their definition of perfection is Gods definition. Look at David, Samson, John the Baptist, Isaiah, Moses. None of these guys fits my ideas of perfection or even acceptability, but then I am not God. Take the apostles, Peter, head strong, Paul, dogmatic, argumentative. Yet these people are called Gods people, holy, righteous, Gods delight.
Now the reward for sharing Gods revelation I am called a legalist, when nothing I have shared has said this. Obedience is about seeing what is obviously right and doing it. Only blind people say doing what is right is evil or legalism, because they do not see it is sin, destruction and death.
This is no idle dispute or the words describing me neutrally. If you think I have made a mistake please show me.
I council about getting real and looking at your own life. I testify to what I know, what I have seen in other peoples lives and what scripture declares. If you say you know the Lord, then tell me my error. Where is my sin? What have I got wrong?