How to treat minor body rust of a car which might be exposed or under a blister?

I have a 1999 Altima and this year I have some minor body rust problem showing around the tires area. Some of the rust is exposed as paint peeled and some are showing as blisters. The average area of the rusty places is 1 inch x 1 inch. What is the best way to treat such rusty areas so as to preserve them and to prevent their expansion? complete rust protetion was done three times for the car, the last one was two years ago. Thank you