Causes of typical rust hotspots in Jimnys 3

Has anyone ever wondered what are the actual causes for several typical rust "hotspots" in Jimnys 3?

The purpose of this topic is to try to determine the causes, so that those who try to apply preventive measures against rust can make the most effect from their efforts.


For example, it is a common knowledge that the body section behind the rear head lamps tends to rust, and it is relatively well known why (cavity behind the headlamp serves as water drain route and unfortunately it rarely drains off completely).


However, I am most puzzled by typical severe rusting of the following sections:
1. Rear floor (below rear seats)
2. Boot
3. Cavities behind/below rear vertical lamps
4. Chassis body mounts


1.
I have observed in several cases that the floor tends to rust the most (and probably first) around the holes through which the rear seat belts anchor to the floor. I noticed that the bolts anchoring them actually protrude completely through the floor and into the open space in the underbody.

My theory is that the moisture creeps in the threads of those holes, destroys them and then proceeds into the interior side of the floor.
Wouldn't then a simple preventive coating of the protruding parts of those bolts and their surroundings from underneath with some sticky waterproof grease prevent the rust from ever gaining a foothold there?

I also noticed that the rear floor tends to rust heavily near the connection with the rear inner wheel arches. Is that caused by some sealant failure on the joints between those two body panels from the outer side?


2.
I have no proper idea what causes the boot floor to rust, especially since it is above a hot exhaust muffler, so it should be regularly dried from underneath whenever its outer side gets wet. I also don't think that any water gathers in the boot floor, unless you have a failure in rear door seals or you are lazy to close a soft top Jimny's roof on rainy days.


3.
I noticed severe rust in (quite inaccessible) body cavities behind / below rear vertical lamps. Those cavities can only be accessed when large rear lower side plastic trim is removed AND then when the rear seat belt winders are removed. Even then, access is quite restricted. I saw quite a bit of rust there even on Jimnys which were only 5 years old (and thus technically should be reported to Suzuki under warranty).

What could be causing rust there?
I do suspect however that the rust in the boot floor is actually an "extended" foray (advanced / later stage) of rust from these parts of the car!


4.
Any idea why rust loves living in chassis to body mounts?



Additional considerations:

Since a lot of rust hotspots are located above the fuel tank and above the rear exhaust muffler, could a contributor to rust's formation be the fact that those body parts are quite obscured by those objects? Obscurity leads to much less air flow (slower drying) and much lower chances accumulated dirt being washed off during regular vehicle washing.

However, on the other hand, the body sections which are obscured by the gearbox and the transfer box do not suffer the same fate ...

All (most) norwegian importers spray stuff under new cars in Norway. We do see the rust in the same places as in other countries but very seldom as bad as some of the pictures i see in here.
When i bought my car it was low milage and was treated extra with underseal in addition to the new car treatment but still I sprayed here and there and treated it especially in the areas you mention. Now 4 years later some rust is forming here and there. Mostly in areas i have not got around to do like front floor of the car and under sills under the plastic.
The rust battle is never won, just postponed..

I can't answer any of your questions with any certainty. Fortunately I identified before purchase from those I inspected that the Jimny exhibits a surprising amount of underbody surface rust by comparison to other manufacturers vehicles of the same age.
With this in mind I purchased the newest and most " rust free" example I could find, planning to rectify and rust proof after after purchase.

So late 2017 I ended up with a 2009 example with 38000 road miles (completely Standard), so at the time of rust proofing it was about 9 years old.

So what did I find:-

In general chassis surface rust had started predominantly on welds. It just looked like the paint / primer finish had just flaked off.
Now to the the front lower chassis mount behind each front wheel, here the under seal protection on the leading edges had failed, resulting in hidden rust in this area.
Now the body.
Surface rust was starting on the sill edges, also under the bonnet front around the radiator. This appeared as a result of lack of stone protection/ chipping the undercoat finish.
Surface rust in a few locations on the lower joint edge, rear floor to inner arch, again suggesting it was stone chip damage to the paint edge.
Below headlamp mounts, passenger side had a hole that required welding. This appears to be poor design coupled with poor protection. It's a drain route though a multi skin metal sandwich with no visible seam sealant. Interestingly the drivers side was ok, suggesting poor quality control on the applied finish or just luck.
A small patch of rust was visible in the boot tool location above the exhaust box, but it appears to have started in a double skinned area with no seam sealant.

What can I conclude.

Suzuki only apply stone chip protection in very targeted areas, I guess it's a way of saving money, and these areas are only just covered.
Excluding final finish over spray, the main finish of none visible areas is primer only.
They do not use any inner body cavity wax.
As per stone chip seam sealant is only used in very targeted areas, other areas rely compleatl on the primer.

Whilst the above may be true of all manufacturers, Suzuki appear to have take it to the limits on the Jimny.
I can only compare to the other vehicles I have owned, all of which have had better levels of protection and the majority of seams fully sealed. They have also been older, had considerably less surface rust and certainly no rot holes in the body.

Not as much fun to own and drive though

Do the other vehicles in the Suzuki range have similar issues or is it a Jimny thing?

Edit - I believe the Jimny is designed to be a japanese kei car, as such is this the reason for the reduced level of protection compared to the rest of the range / other manufacturers?

There's enough waxoyl under mine to make the exxon valdez disaster look like a kids birthday party, this was the first job I did on getting mine and tbh I'm still at it every weekend. I'm yet to pop the plastic off the doors and arches but I will get in there before it gets too damp and wintery. When going through the process of doing the area around my tank I noticed that mud and other schmoo had accreted on the top and I had to spoon a load of that off before I could get in with the black stuff, no good at all but I will not see my jimmy succumb to the cornish tin worm it took me ages to find a jimmy that was 'good enough' underneath to pass muster.

I have spotted an issue with Ulysses's A/C compressor being constantly engaged (clutch rusted on i think). This is making the heater matrix area wet with frost, which then drips down into the passenger footwell and soaks the carpet. This is the reason the passenger footwell was worse than the drivers I think.

It is nice and cold in the car however