CSI: rear wheel arch rust. Seal these fasteners!

[foama]

This car war treated with cavity wax in every cavity when it was new in 2014, and five years later again.

With help of an endoscope I checked the wheel-arch cavities today, and found them clean, dry and with zero rust.
Just to make sure it stays that way, and prompted by the pictures in previous post, I sprayed them again. See photo. The tube of the cavity spray is inserted all the way, being about 35cm (14 ins.) inside the car to reach the tip of the arch.
If water does get in, it is indeed a water trap. The rubber grommet for the rear lights as well as the fasteners were removed to apply some silicone grease before putting them back in for preventing water ingress.

[MetroMPG]

That's ideal, foama.

I'm slightly doubting my original diagnosis of how the water is getting in, though:

After I posted this and put the tail light back on, we had an afternoon of heavy rain. I checked the cavity the next day, and it was bone dry.

So then I aimed the garden hose at the light and sprayed into the gap for a minute or so... and the cavity was still dry!

I have to take that garden hose and aim it at the other fasteners.

[twister77]

I'm slightly doubting my original diagnosis of how the water is getting in, though

We should also cosider that for the cars which spend their lives in certain climates, any condesation inside body panels will find its way to the bottom of the panel. If there is no way out for that condensed watter (like this cavity formed between the outer wheel arch sheet metal and the inner wheel well structure), it will start to rust from the inside.

[MetroMPG]

Condensation inside the panel would mean... cold outside, warm & humid inside.

I guess it's theoretically possible (a lot of heavy breathing in the car on cold days?), but I'd be surprised if that were the case in this particular panel, at the very rear of the car, in a cargo compartment separated (more or less) from the cabin. Also I know for a fact this car was a 1-person commuter mobile.

Back to my original diagnosis:

I definitely saw water getting inside 2 of the 4 fasteners when I squirted them from a low pressure squirt gun. That's why I was very surprised I saw no water get in when I aimed high pressure & high volume in the gap between the light & body.

[MigWay]

Tried to spray oil heavily from inside by pulling out plastic trims a bit.
After a day of sun exposure...which lowered the grease viscosity... it is dripping between quarter panel and rear bumper
did visual inspection...no evidence of hole, rust, paint chips and what so ever...
This is pretty shocking! I cant help but want to say thank you to this thread so I am able to prevent a foreseen issue.
In short, factory sealant is either not enough or not withstanding cold weather.

[MetroMPG]

That's surprising - are you saying you filled the void, and it began leaking out from another point? Can you tell exactly where the gap is, or is it hidden behind the bumper cover?

[MigWay]

That's surprising - are you saying you filled the void, and it began leaking out from another point? Can you tell exactly where the gap is, or is it hidden behind the bumper cover?

Exactly! I don't see any gap. It is around the edge around quarter panel and rear bumper cover.
The product I used was Rust Check Coat and Protect
In this case, I would dump heavily cosmoline and let it dry to build up a barrier.

[foama]

Tried to spray oil heavily from inside by pulling out plastic trims a bit.
After a day of sun exposure...which lowered the grease viscosity... it is dripping between quarter panel and rear bumper ...

When I liberally sprayed that area from inside, there was no dripping at all. Nothing whatsoever!

You may want to check that closer, something seems not good.

[MetroMPG]

It could also be sealing inconsistencies from car to car.

My Mirage #2 (red) has only a tiny fraction of the rust in this area that #3 (white) has, despite being the same age, and being driven in the same region.

[MigWay]

When I liberally sprayed that area from inside, there was no dripping at all. Nothing whatsoever!

You may want to check that closer, something seems not good.

I was spraying from luggage compartment inside the plastic trims. Not from the rear tail light wire harness plug.

Seems to see a tiny cavity leaking out grease if you enlarge the third pic I took.