1. Get yourself a can of the Hammerite 3-in-1 paint. It has it all on one : rust inhibition, primer, and a resilient resin-like coat. For max. aesthetics, make sure the Hammerite color is as close to the color of your car as possible. Btw the color choices are not that wide with Hammerite, unfortunately, but my cars are black, so black on black looks more or less OK.

2. Mechanically clean off as much visible rust as you possibly can.

3. If you see that rust goes underneath your car paint - mercilessly peel off pieces of your car´s paint until you are perfectly sure the whole rust is exposed. To give the peeled-off area a cleaner look, you may want to first make a cut through the paint with a sharp knife around the whole area which you plan to free from paint.

4. Mechanically remove old primer and all the mechanically removable rust you have exposed by removing paint - you want to get to the bare metal. Important: if some non-removable rust still remains on the metal surface after all your cleaning efforts (and it probably will remain), it is NOT a problem: the Hammerite rust inhibition chemistry will deal with it.

5. Cover the paint-free cleaned area with Hammerite and let it dry.

Hammerite is also good for sealing paint damage done by stones - it holds and lasts and seals so much better than commercially available paint chip repair kits.

Out of experience, this whole rust curing process takes like 2 to 15 minutes (without drying time), can be done in the street, and I have never had to get back to an ex-rusty spot once cured this way yet.

Obviously, the fix only works for rust which is only present on one side of the metal. If the corrosion is deep and went through the metal to its other side, way more serious and totally different treatment will be required.