Best rust treatment i know of is cold galv, but kill rust does the same job with a gloss finish, dont know if it comes in a spray though. I usually clean off rust spray cold galv then with a matching gloss
Another spot for rust is the engine bay chassis rails there is a folded sheet that runs along the top and rust gets between seen this on a import that was rejected because of it. They cut out the chassis no. and took all the id plates.