Reasons for rust

Some people say that Citroën was the inventor of rust. This isn't totally true - FIAT also claims to hold the patent for rust. ;)

The 2CV was not designed to prevent rust
You have to keep in mind, that the basic design was made in the late 30's. At that time nobody thought of rust problems. Still it took long to prevent rust from the design already. This started only in the 70's. Today's cars should have no places where dirt and water can be collected. In the 2CV all these places permanently kept the humidity on the metal. The result is rust.

Lousy material was used to make the 2CV
The rumor goes that after the war most of the steel was recycled with lots of scrap metal. So the quality of the steel was poor. Therefore the cars built in the 50's and 60's in Europe were not resistant against rust.

The 2CV was designed as a cheap car, not made to last long
When Pierre Jules Boulanger did one of the first market researches in history, he found out that the people needed a cheap and reliable car with a good fuel economy. So the basic requirements for the design were that the car should be affordable for an average income.

Lousy quality
The 2CV had to be cheap. In the beginning this concept worked out. Over the years the production of cars got more sophisticated. The new production plants worked more and more automatically. Robots assembled and welded the cars. This of course requires a design that allows the robot to do its job. The 2CV dated back to a time when an automatic production was seen as science fiction. All the work had to be done manually. This increased the production costs for the 2CV. So they had to cut back on the costs. The result was less material and quality. Over the years the metal became thinner and thinner. For example the sheet metal used for the frame was only about half as thick as the material used in the older 2CV's. Also less primer and paint was used. In the last years of production on the inside of the car you could sometimes see the primer trough the paint!
The biggest cutback in quality happened when Citroën moved the 2CV production to Portugal for the last three years of production. The quality we got from there was absolutely lousy. A 2CV that was used everyday and that did not receive a good rust protection in the beginning, was unsafe to drive after 3 years already!

There is tons of salt on the roads in wintertime
Whenever the first frost and snow is expected, the road maintenance crews start to throw tons of salt over all roads, especially on the freeways. The asphalt turns white from the salt, so do the cars.
The salt gets everywhere in the frame and all over the body. It will stay there a long time. As long as there is still salt left, it will keep the water and corrode the steel.
So all unprotected spots are endangered. This is mainly true inside the frame since there is almost no paint and no primer. The salty water will creep between all the panels in the frame and work on their disintegration. This photo was taken after about 60 mi on the Autobahn. Before the car was washed and clean.

The roads are white from salt - so are the cars

Standard wax doesn't work
The wax that is used for the standard protection will not really help. This material is good for a flat surface, but the internal structure is far away from this picture. The stability comes from the horizontal reinforcements. They are spot welded to the top and bottom panels. The wax will not get between these panels, since the space is not big enough. The protection with wax is good for some time, but after a few years the wax will dry out. The producer will promise you that this never happens, but the experience tells a different story. When you move the car, the whole body flexes. This makes the hard wax to get very fine cracks. The saltwater is now actually sucked through the cracks between these structures. When this happens, the wax even works bad for the car. It will prevent the water from evaporating. So the saltwater can work on the uncovered metal.

The panels were not painted with primer before they were welded
All the material used to build the body was not painted with a proper primer. Then the welding was done. Many connections between panels were sealed off with huge amounts of a rubber sealant.
After some years condensation will take place between the rubber and the metal. This will destroy the unprotected metal in a long term. This is the reason for the rust over the aluminum trim over the rear fenders. Also the rust spots what you will find next to the taillights are caused by this.

The 2CV's suffer from poor maintenance
Since the 2CV was seen as a very cheap car, the owners didn't care much for maintenance. Mostly the cars were not washed and waxed. Even worse, also the technical maintenance wasn't done. Most cars were just been driven until they broke down. Subsequently the 2CV got a bad reputation in reliability. This attitude only changed a few years after the production was stopped, but that was too late for most cars.