Seam sealer locations

[A-man930]

Once again, my slow pace takes its toll...

Where did the factory put seam sealer in the engine bay, exterior, and interior? I'm 75% confident in what "makes sense" when looking over everything, but I'm not interested in missing some weird path for water to find its way in.  

 

 

 

 

[89 MJ]

I’d put it on every seam you can see. Even the factory didn’t get them sealed up perfectly

[Boilermaker]

Seam sealing locations from MR278.

1571325217_MR278R11-22.pdf

[Gjeep]

I agree. Seam seal everything. That's what I did before paint, especially floorboards. 

[A-man930]

4 hours ago, Boilermaker said:

Seam sealing locations from MR278.

1571325217_MR278R11-22.pdf 2.09 MB · 4 downloads

 

You rock. Thank you! 

[Boilermaker]

No problem. As the others have said, seam seal everything. Only one other thing to add, and from your pics looks like you may have already done this - remove all the factory seam sealer you can. When I had my truck cleaned up, most of the spots that needed attention were under the factory seal sealer.

[A-man930]

I also discovered rust under the factory sealer.  Spooky.  

 

It seems there are different schools of thought regarding seam sealer, and I think the "seal one side" approach makes the most sense.  

As appealing as "seal everything" sounds, you have to ensure that moisture isn't trapped.  I won't pretend that you can actually prevent moisture from getting in.  Even condensation from hot/cold cycles is enough to cause problems.  

 

It looks like Jeep thought the same way:

 

 

Edit:

 

I do plan on being very liberal on the "one side"

[Boilermaker]

I think all of the sealer from the factory is 1k which often shrinks as it dries. Probably more so where its applied the thickest, like the front of the floor pans and back of the interior cab. So what was likely done to protect the metal from the worst areas of water penetration may have made things worse in the long run.

 

[A-man930]

8 hours ago, Boilermaker said:

I think all of the sealer from the factory is 1k which often shrinks as it dries. Probably more so where its applied the thickest, like the front of the floor pans and back of the interior cab. So what was likely done to protect the metal from the worst areas of water penetration may have made things worse in the long run.

 

I'm going with a combo of brushable 1K and some 2K.  Not sure exactly where each will be applied.

 

[ghetdjc320]

The factory tended to seam seal the back side of joints (the non exposed side). I really have mixed feelings about that. You can’t leave places for moisture to accumulate without proper drainage. At times, seam sealer can actually trap moisture or prevent drainage. That’s why I like POR15 inside metal seams such as pinch welds followed by seam sealing the back side. The por15 loves to seal up cracks and crevices in metal. The process I used was to mask off the back side of a seam then allow some POR15 to wick its way down int the joint. I’d the seam sealer the back side of the joint then hit both side with raptor liner. Did most of my truck that way. Strategically adding drain holes in certain areas before this process helped a lot as well. And one quick tech tip for removing old seam sealer, use an oscilating tool with a caulk cutting blade. They work very well at getting the old single stage seam sealer off. The factory application was horrendous on my 90’. 

[coheed]

I'd also recommend getting every bit of factory sealer off. I had rust under mine all the way up to the cowl on the drivers side. I also did what @ghetdjc320 suggests with POR15, even going so far as thinning some slightly and applying multiple coats to overlapping areas until I saw a little seeping through. I would use 2K anywhere the gap I was sealing was larger than the shrink ratio of the 1K sealer. I only used a 1K, but in areas with larger gaps I made sure to keep the beads fairly flat and overlapping the seam by half the width of the gap-ish, and it still seems fine almost a year later.

[A-man930]

Disclaimer: not an expert. 

 

Whichever side is open needs to allow for whatever gets In to drain / dry out and have adequate corrosion protection (I would think this is generally the side facing the elements).  I'm using 2k epoxy in place of POR15 - I have to give mad props to their marketing department, as they've got lots of people on board with their product...  Anyway, in my opinion, applying a product like Raptor liner on both sides is likely to trap moisture.  

I think the missing piece in your recipe is cavity wax. It may require periodic reapplication in some areas, but it's going to complete the picture.

 

I'm curious as to where you added drainage?

 

[A-man930]

Noticeably absent from the service manual... 

[coheed]

I can't speak for anyone else, but I'm not an expert either. Nor do I think POR15 is a do-all product. I only used it because I had about a half gallon sitting around. It also wasn't the only thing I used. I only used POR15 inside the truck on the top surface of the floor and braces. The thinned out POR15 I used in areas of overlap like where the brace goes in between the floor and the brace at the bottom of the back wall, and some vertical welds on the firewall and trans tunnel. Any cavity I couldn't get a brush into got Eastwood internal frame coating applied. Any seam or butt joint outside the truck got a liberal coat of Mastercoat (I would've done everything with Mastercoat but at the time you could only buy a quart at a time, and it was out of stock more often than not), then seam sealer applied over it. And any surface within 3/8-1/2" of a weld got 2 coats of self-etching weld-through primer. The only areas where I thought moisture could get trapped got Mastercoat over the weld-through primer, and I put it on with a small brush and really tried to jam it in there. Time will tell if that was the right approach I guess.

[A-man930]

On 3/19/2026 at 8:50 PM, coheed said:

Time will tell if that was the right approach I guess.

 

This. 

Aside from observing the current state of the OEM's work from 35ish years ago, how often do we really know the effectiveness of all this stuff over 10+ years of regular use?  

[ghetdjc320]

7 hours ago, A-man930 said:

 

This. 

Aside from observing the current state of the OEM's work from 35ish years ago, how often do we really know the effectiveness of all this stuff over 10+ years of regular use?  

Also, where the MJ lives and is stored will make a huge difference. Spending its life garage stored in a dry desert climate will not reveal the full extent of durability or lack thereof from the factory products/application.

I can say with some degree of certainty though, that the work spent sealing up metal joints will really help prevent moisture related problems down the road. it’s pretty easy to improve upon the factory work. I’m sure there are build quality variations between each MJ  but my 90 had abominable quality workmanship. The seam sealer, the spot & mig welds, the gaps, the accuracy of bracket placement was just atrocious. My 91 had significantly better build quality overall. I love these Comanche’s but it seems like they were built as cheap, disposable beaters. It’s not terribly difficult to improve upon many aspects of the factory workmanship/materials and the seam sealer is most certainly one of those areas. And I’m not knocking it for being 34+ years old at this point, I’ve worked restoring several much older vehicles with build quality easily exceeding that of the Comanche. The engineering behind the MJ seems decent but the assembly work 

[ghetdjc320]

On 3/19/2026 at 5:59 PM, A-man930 said:

in my opinion, applying a product like Raptor liner on both sides is likely to trap moisture.  

I think the missing piece in your recipe is cavity wax

It depends on what you’re using the raptor liner or similar product for. I was using it as a replacement for undercoating. Raptor liner is a good product that doesn’t absorb moisture and is appropriate for certain applications if the TDS is followed. POR15 is a moisture cure product which also has its applications if applied as specified in the TDS. And yes, I used plenty of epoxy primer in the right settings. All my finished body panels received several coats of CRE epoxy primer before even getting any fillers or glazes. I’d follow that with a reduced CRE epoxy to use as a sealer.  In areas such as rocker panels and inside uniframe channels I applied a few coats of Eastwood internal frame coating which is a very thin coating. Followed that with 3m cavity wax. Also used pillar foam in different varieties in gaps between the skin and sub structures. I can’t say it’s the best method as I’m sure there will always be some new product on the market that may work better. But it worked very well for my MJ in some brutal weather conditions.