Trunk Floor Crack/Separation

[72MetallicMalaga]

I've developed a pretty noticeable separation in the corner where the spare tire trunk floor meets the left rear inner fender.

 

Has anyone else had this occur before? I'm assuming this is a repair requiring welding...

 

 

[TobyB]

Oh, #waffles, no, don't weld that!  

 

You'll kill your paint!!!

 

So this was an early attempt at 'panel bonding'- the trunk floor was hung off the OUTER

fender skin with an adhesive, as any welds would show up as distortion on the outer panel.

 

It was successful in the normal lifespan of the car, but after more years, the adhesive shrinks and fails.

If there's no rust in there, get some urethane adhesive that sets solid (not stays sticky)

and fill the gaps, removing as much of the old material, debris, and mouse turds as you can.

It doesn't need to be super strong, but it does need to stay relatively flexible.  Your local Auto

Body Supply can give you a tube- don't be shocked if it's used for installing 'structural' windshields.

 

Normal panel adhesives for modern cars won't work, as they count on close contact, high assembly pressure,

tight fits, etc etc etc. to really work.

 

That said, any non- corrosive (don't use silicone) adhesive ought to be fine.

 

This comes up from time to time, but not all that often.

 

t

glue it!

[72MetallicMalaga]

19 minutes ago, TobyB said:

Oh, #waffles, no, don't weld that!  

 

You'll kill your paint!!!

 

So this was an early attempt at 'panel bonding'- the trunk floor was hung off the OUTER

fender skin with an adhesive, as any welds would show up as distortion on the outer panel.

 

It was successful in the normal lifespan of the car, but after more years, the adhesive shrinks and fails.

If there's no rust in there, get some urethane adhesive that sets solid (not stays sticky)

and fill the gaps, removing as much of the old material, debris, and mouse turds as you can.

It doesn't need to be super strong, but it does need to stay relatively flexible.  Your local Auto

Body Supply can give you a tube- don't be shocked if it's used for installing 'structural' windshields.

 

Normal panel adhesives for modern cars won't work, as they count on close contact, high assembly pressure,

tight fits, etc etc etc. to really work.

 

That said, any non- corrosive (don't use silicone) adhesive ought to be fine.

 

This comes up from time to time, but not all that often.

 

t

glue it!

 

Got it! I'll try this out, thank you so much!

[esty]

this is good stuff but as said, you can by an alternative at most corner stores

por+putty - Google Search

WWW.GOOGLE.COM

 

[PhilC]

That's very interesting information 👍 BMW ahead of the game.

[Ian]

I use POR15 epoxy putty a lot and it will fill gaps.  But it dries hard and inflexible.

Edited January 12, 2023 by Ian

[dlacey]

This is a job for Seam Sealer:

https://www.3m.com/3M/en_US/p/d/b40066999/

 

[jp5Touring]

Agreed ,  I would use a quality Seam Sealer like 3M.  I can ask my Son what brand they use at the Restoration Shop he works at.   He complains its stronger than welding.  

[Ian]

3 hours ago, jp5Touring said:

Agreed ,  I would use a quality Seam Sealer like 3M.  I can ask my Son what brand they use at the Restoration Shop he works at.   He complains its stronger than welding.  

I'd bet very interested in his favorite high-strength seam-sealer.  I tend to use a lot of POR15 Por-Patch as a seam sealer, but it's not very flexible. 

[jp5Touring]

They do high end muscle car stuff.

3M  08310  for most things.  I'll have to check if okay for painted surfaces.

https://www.3m.com/3M/en_US/p/d/b40066449/

 

3M 05887  if you need to sand it. They say it turns out very nice.

https://www.3m.com/3M/en_US/p/d/b40071609/

 

His work,, proud Dad moment.  🙂

 

 

 

 

[Ian]

Your son does nice work!  

 

Thanks for the seam sealer advice. For most of my work, I use POR-Patch and squeeze it out of the tube like toothpaste and then work it into the joint with a gloved fingertip.  Works great for rust-proofing welds and joints and makes seams water-tight, but it's not flexible.

 

Edited January 12, 2023 by Ian

[Mike Self]

The stuff in that joint is more sealer than adhesive...I'm sure there's a reason that BMW didn't actually weld the (metal) trunk floor to the quarter panel, but they didn't, thus leaving a 6-7mm gap between the two panels.  And has been pointed out, as the OEM stuff enters its second or third decade, it hardens, then pulls away from either the quarter panel or trunk floor, allowing moisture to creep in.  And since the stuff was applied before the car was painted, what's underneath is bare metal, so it's gonna rust.  Worse if the trunk leaks.  

 

If yours is cracked or coming loose, you should scrape it all out (a curved linoleum knife is good for this), use anti rust paint on the now visible naked metal, then re-seal with and adhesive, flexible sealer.  You might want to do the same thing to the same stuff the factory applied along the inside bottom edge of the doors, for the same reason...

 

mike