Best way to treat it (given the location and access) my opinion would be:
1) Sand down the area of the rust and around the area also (this helps to prevent it spreading) use an aggressive sandpaper like 400 grit
2) when you have sanded it down you will have removed the surface rust, after a little while you will notice a lot leaves, but you might just have a few rust spots. Use a product called KRUST (available at euro car parts) and just follow the instructions. I would suggest given the size of it, apply a minimum of 3 layers allowing to dry inbetween.
3) buy spray primer (any will do) and tape off the area to protect from overspray. Spray the area and allow to dry, apply 3 coats of primer to give it a good bond.
4) buy a tin of spray paint for the colour match of your car. Apply three layers of this allowing to dry inbetween, If it dries uneven, use a fine course sandpaper (say 2000 grit) to rub down the area and smooth it off once its dried.
5) Once its all on there nicely, then you need to apply a spray lacquer. Again, I would go with 3 coats to ensure the best protection.
Once you have done all this, the best thing to do is try and blend it in, depending on how harsh the paint spray lines are, depends on the course of action. if its barely noticeable, something like T Cut should help you to blend the two together, otherwise a more aggressive rubbing compound may be needed.