With 320,000 KM and 7 years any prelimary rust damage has been done..... with a little bit of rust showing on your brick, i would suggest saving your cash than wasting it.

Once your car starts rusting the ONLY way *and i researched it* you can prevent any further rust is take it to an autobody shop and have them go through the ENTIRE car and remove any rust spots, which is an expensive fix for an older car. And still you dont get a gaurentee that your car wont rust

If you rustproof a car that already has a rust spot on it, you are only covering the rust not gedding rid of it, save the cash. Call the volvo dealer to see if your year volvo went through a galvanized bath during production, that usually keeps the car frame from rusting.