What do you mean about the car as a whole, as in some form of durable wax or sealant? Another option you have for the exposed rust is using cavity wax to act as a temporary barrier especially if the rust more severe than simple surface rust. If you have plans to have the rust repaired, POR15 would be an awesome solution for the time being but as long as the rust is still underneath, it will always be a matter of time before it comes back to the surface again. 