Rust has drilled holes in that 2CV flap.

The good way for stopping rust under the windscreen rubber seal should have been to remove the seal. Here, only exterior rust was poorly repaired. It has been completely useless : rust continued to attack. Notice too the rust around the spot welds along the hinge of the hood : imposible to slide and remove the hood any more, plus ingress of water inside the dashboard area through the rust holes.

For those two 2CVs, leaks of rain inside the car for months had destroyed the chassis.

At the rear of a 2CV, around the stop lights.
The rust in that area comes from a bad rubber seal around the plastic light cap.
Rain cannot leak to the chassis from this place, but if the body has rusted here, it has probably rusted elsewhere, may be inside the chassis.
Conclusion : beware... and go on inspecting carefully such a 2CV before to buy !

Rust is the genetic ennemy of the 2CVs.
It can totally destroy a body or a chassis within just a few years.

Very often rust on the body of 2CVs is let as is because 2CVs are not considered worth to be maintained. There are so many of them here in France, it is not worth to stop rust when it appears or professionnaly repair large rusted areas. The fact is there are a lot of 2CVs here around. Buyers are mainly young people without money because who else in France would like to drive such a slow car ? As they have no money, they will prefer to buy a cheap rusted 2CV than a well maintained one and so, it is not worth to maintain a 2CV because you will loose your money as nobody will buy your 2CV the right price (that is a vicious circle...).

Sometime, owners repair rust by themselves... and usually just add some glass fiber and paint over the rust which continue its horrible work under the paint !

When buying a 2CV, you have to pay attention to several specific areas : around the windscreen then inside the car, on the floor at the front for driver and passenger sides.
If you can see rust which has drilled holes in the metal in one of those area, that is a very bad sign ! On nine 2CVs upon ten, you will have the chassis rusted too, ready to collapse.

The explanation is as follows :
Rain leaks inside the car through the rust holes around the windscreen (or sometime through a bad adjusted flap).
Water goes under the rubber floor mat. It rusts the floor, on both sides and also rusts the top of the chassis (which is the floor in the middle of the 2CV).
If you are lucky, rust goes faster on the sides and drills holes. This protects the chassis because the leaks of rain are evacuated outside the 2CV through the rust holes, protecting the top of the chassis. This, unfortunately, happens only on one 2CV upon ten...
If you're not lucky, rust goes faster on the top of the chassis (which is usually less painted than the floor...), drills holes, water invades the interior of the chassis then rust comes. The chassis is badly corroded from inside.
Usually, the chassis slightly bent but do not collapse completely.

You can detect a slightly bent chassis from different ways :

Check the tube with the headlights in the front of the bonnet.
Does the tube touches the bonnet ?
If yes, the chassis is probably bent, unless bumper/fenders have been badly bolted, or damaged and badly repaired.

Check the triangles between the bonnet and the front fenders, on their straight side where they touch the main body.
Do you have enough space at the bottom to put a finger inside and no space for that same finger at the top ?
If yes, chassis is more or less bent, unless fenders/triangles have been badly bolted.

Look inside the front fenders, on the arch for the cardan from the gearbox to the wheel.
Between the cardan and the arch, do you have all space at the front and none at the rear ? The bottom of the arch, at the rear, is slightly bent by design but was the arch modified (frankly twisted or cutted out) because it was touching the cardan ?
If yes, chassis is more or less bent.

Look at the bottom of the steering column. (remove rubber if necessary).
Is it not centered on the hole of the floorpan so much that it is touching the floorpan or touching when you turn the column ? Is the column itself bent on its middle ?
If yes, chassis is more or less bent.

There is something easy to do for fixing humidity and rust problems. We commonly do it here in France to protect a 2CV. It may appears horrible to you but believe me : that is a good solution.
Here it is :
Drill 3 holes (diam. min. 7mm) on each side of the floor, on both the passenger and driver side. Drill the holes on the flat area in front of the seats (that is, where water naturally stays). Drill them in the stamped wide lines made to enforce rigidity of the floor. Drill one close to the door, one close to the chassis and one in the middle. This may sound a sacrilege, but with holes drilled as indicated before, water and humidity will immediatly go out of the car without any further damages. More : those holes will help the floor to "breathe" under the carpet and to stay dry.

Remember this advice, my friend, it may help you save time, money and avoid frustration :
You can dream of a 2CV from pictures but, as far as possible, do not buy one without seing by yourself the weak areas, or, at least, without asking for close-up pictures of those areas.

Now, if you are really on a budget but have a lot of time and a deep will to acquire know-how about bodywork or bolts and nuts, then you can learn the hard way about rust in a 2CV body or 2CV chassis.