Rust

[Codey]

How can you stop rust so that it wont come back. Mines rusting out behind the rear wheels under the drivers side door and there's a big hole and little rust spots all over.
i tried sanding it and bondo over it and painting over the bondo but its starting back up how can i stop it for good.

 

[Old77]

You can't actually stop rust but merely slow it and bondo over rust is definitely not suggested. Proper rust repair would include cutting out the cancer, patch the area with patch panel(s) and perform proper body work and applying a good undercoating.