F12 Water trapped in back hatch

Sorry if I missed this somewhere in the thread, but the corners you tape when you wash are the top corners of where the trunk lid meets the bodywork?

 

No its the top corners of the 3rd brake light over the rubber seals on the trunk lid.

 

So upon inspection, it didn't seem like my car had the same gaps in those channels, however I may just run a small amount of RTV over the area to keep it sealed.

 

I know this is an old thread but I felt it worthy of an update since it’s a useful resource for hatch leak problems. The problem for me occurred to such an extent that water would pour out of the tailgate every time I opened it (at the joint between the side section and the rearmost panel). The third brake light bar had been replaced due to a faulty LED segment and the problem had occurred after that. So it was time to disassemble.

What I found is that the brake light bar has had its construction upgraded to increase the area protected by the foam surround.

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Unfortunately, the foam was compressing so much on one side that it had skewed and was leaking.

I didn’t take a photo of it while it was completely compressed and it has since regained a normal profile but you can see that it hasn’t recovered completely.

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The water leak was only on one side and it was only one failed gasket but it was enough to significantly wet the inside of the car when the hatch was opened and the water came out.

So what is to be done? I could caulk everything up with sealant as others have done but it seems to be too permanent and if I don’t get the fix right first time, there will be no second chances.

So I’ve decided to use O-ring grease which is rubber safe and resistant to water, moisture, evaporation and is heat stable.

I’m going to grease up all the gaskets and the weatherstripping as well as fill the top corners of the hatch window weatherstripping with grease.

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The one area in which I’m going to use some silicone filler is the light bar itself.

The light bar in my car sits on top of the window weather strip (unlike the photos of some others on this thread and I think it forms a good seal.

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However, there is a gap where water can get in at the corners and it’s caused by the shape of the light bar.

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So I intend to put a fillet of sealant in this area in order to eliminate the gap before mounting the light bar with more grease to the car.

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Thanks, please let us know how your repair works.

 

I will do, thank you. We have a trip to the Alps planned hence the urgency to fix the leak. There’s always rain in the mountains so the repair is going to get a good test.

The silicone fillets have been applied - masking tape still on.

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Masking removed - now the silicone needs to cure for 24 hours.

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In my case, I think that what has happened is that water has run off from the highest point of the hatch from the centre-line of the car, down the weatherstripping to the corners and then under the light bar through the gap in the corner (which I’ve now filleted so hopefully it will stand up to heavy rain and even jet washing).

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I've been puzzling over why some people get small amounts of water in the top of the tailgate while others get water at the bottom and I think I've worked it out. All the water leaks come in from the light bar. I don't think that the rubber side channels round the glass of the window are letting any water in - and any which does come in drains out of the bottom like @rfg in post #67. So what has been achieved by people sealing the top corners of the window weatherstripping is that they're preventing water from coming in under the raised bit of rubber and entering the void under the light bar.

Now, to my theory of why water ends up in different places.

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You can see the four bolt holes circled in blue. If water leaks through there, it ends up in the top of the tailgate.
But there are four other holes (circled in red) which lead into the frame of the tailgate. If water gets in there, it travels down the side of the tailgate (red arrows in next picture) to the bottom of the tailgate where it leaks out into the rearmost trim. And when you lift the tailgate, it all comes pouring out of the trim.

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I've been suffering from water pouring out of the bottom of the tailgate when I open it and when I removed the light bar, the wet area was equivalent to the holes circled in red in the first photo.

 

I had the dealer do the bulletin which uses sealant in the corners. But the real fix is 1) don't drive in the rain and 2) use the blue painters tape over the rubber seals around the third light when you wash it. The tape is also used around the rear mesh so there's no dielectric required for galvanic corrosion.

 

So the light strip has been reinstalled and survived the jetwash. The forecast for the alps/dolomites this coming week is for biblical rain so I hope the fix holds but I’ve taken @Solid State ’s advice and packed some masking tape just in case…

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