It depends on the source of the rust, and your definition of the word “fix”. There are basically 2 types of automotive rust; surface rust and structural rust. Chemically, rust is simply iron ore, from which steel is made, combining with oxygen and returning to its natural state. Rust can be prevented by keeping oxygen from contacting steel.
Surface rust comes from things like stone chips and scratches that expose the metal to oxygen.This type of rust is repairable. The paint damage is reversed, and the rust is permanently fixed.
Structural rust comes from the design of how auto bodies are put together. Steel panels are joined together by welding, of which the most common form is the “pinch” weld. The basic flaw here is that pinch welds will not be successful if there are any contaminants present (such as internal rust preventatives) where the weld is attempted. The best that can be done, after welding, is to attempt to coat the weld by either dipping the part in a rust preventative, or spraying on a product called “E-coat” (electro-statically charged primer coating). Auto manufacturers have only marginal success preventing rust. Neither of these processes are practical after a car has been assembled. The cost would exceed the value of the car.
So now let’s define “fix”. What other shops sometimes sell as “fixing rust”, I call “putting a band-aid on cancer.” If you mean cut out the metal around the brown holes and weld metal in its place, this indeed can be done. However; this is only, at best, a temporary repair. This process has in fact multiplied the areas where steel panels are lapped together, with little or no chance of rust preventative coatings sealing out contact with oxygen. Instead of “fixing” the rust, they have effectively accelerated and enlarged the problem. It may look better for weeks or a few months at best,but the natural processes occurring underneath will soon displace the fillers and paints that cover such areas.
Personally, I do not “fix” rust on my own personal vehicles. Why? Because even when I can “fix” my rust for free, it works against me in the long run. I waste time, materials, and ultimately I am accelerating the problem.
The only REAL way to fix rust is to replace entire components (doors, fenders, etc.) with new ones. If unibody components (rocker panels,floor boards, etc.) are rusted, the cost of replacement usually exceeds the value of the vehicle.