The only way to remove rust is to do just that remove the rust!
Use a grinder and grind back to clean metal. Then check there are no tiny pin holes. If so then you will have to remove the affected area and weld in new metal. This however isn't always enough!!
I speak from experience. Working in a body shop you get the odd old banger but worse than that are the rich boys with really old porkers and prancing donkeys. The cars can be worth a fortune and they get charged a fortune aswell so you have to do a shi1thot job! Very time consuming and boring!
Well so far I've only used kurust on a few bits and bobs and personally so far I've had no problems with. Don't forget that you're not supposed to completely remove the rust with this stuff. it's applied directly to it. It seems probably best not to use it, okilydokily. As i said, it's only surface stuff and am not too worried about it, but i want to sort it before covering it with the 3M stuff which is the dogs danglies.
I would not bother with either. I wasted time trying that stuff. There is no substitute for removing the rust. Next best thing, which can last years, wire brush/grind/sand most of the rust off, use a high zinc primer, Davids, or whatever. Look at the zinc content on the tin. Many are low zinc, as zinc is toxic. Let it dry for 24 hours, perhaps 2 coats, then prime, paint and underseal, etc.
You must paint on top of the zinc primer, as primer is not waterproof. Zinc really does protect the metal underneath.
Always found Kurust o do the job myself. But if you leave it at that then it'll just come back, Once you've rubbed down and kurusted then you need to seal it. The best thing for this if it's hidden is smoothrite. No air-no rot. If it's anything more than general surface rust it's best to cut it out but then seal the new metal before it starts to rust as well.