Why can’t you repair a rusted surface by applying putty and then sanding it?
When you see rust on your front door, it’s too late to save it. When you can actually see the rust, it means that the metal has already corroded behind the paint film. At this point, even after removing the rust, there’s no way to guarantee that it won’t come back. This can jeopardize the lifespan of a paint job and for this reason, we avoid all rusted surfaces.