Cured NCS will fail cohesively before it fails adhesively if applied correctly (they break apart before they unadhere from the surface) but can be scraped off pretty easily with a plastic putty knife, and most can be removed with xylene and scrubbing. One exception would be 1199 which is fairly permanent, bonds to some plastics even. They cure to a softer, more flexible product than acetoxy does, and finish is usually more matte. Definitely easier to remove than butyl in most situations, that stuff takes a lot of rags and solvent to clean, change rags, add more solvent, over and over. They aren't easy to find locally but can be ordered online and will last for years after expiration date if kept cool. But even most of the good acetoxy cures these days are more than reliable enough for most applications. GE makes decent sealant that is carried by Home Depot, I've used that on commercial work over the years if we were in a jam and never had a callback on it. If someone is experiencing failure with one I have to believe it's more likely an installation issue than material issue, or incompatible material like plastic.

As far as static vs dynamic loading, some of these structures we've worked on were 100+' in the air and hundreds of feet long. You can hear the metal expanding when the sun hits it for a few minutes and wind speeds higher than most anyone will hit in a 4Runner aren't uncommon. I don't do much for curtainwalls but their installers use mostly the same products, and they can be 1000'+ up there dealing with even higher dynamic loads.

Update and supplier info since thought I got a couple PMs (and thought I responded at some point but they seem to have disappeared from my inbox and nothing ever shows up in sent folder, so apologies if not, been super busy this year).

I installed a Sherpa rack a few months ago and did end up using skinning butyl (PTI 757). The Sherpa racks, or at least the Quandary for GX470 uses a very hard rubber/EPDM block as a spacer/sealer in channel, below mounting bracket. I'm not confident that this block is able to compress enough to seal out all water, although would probably stop most of it. My paint was dinged up from factory rack bolts which would provide several possible channels for water too seep in, and of course the roof itself is a light radius whereas the blocks were square. IMO for weatherproofing these should've been silicone blocks, more pliable. That might've come at the expense of load capacity although there are metal sleeves through it, around the bolt, so probably wouldn't matter too much. Anyway, silicone won't bond well to EPDM so I used butyl after scrubbing the scum out of channels. If the blocks were silicone I could've used silicone sealant.





Here's a pic of one corner of the custom roof rack setup I installed using Dowsil (then Dow Corning) 795 10ish years ago, it never leaked.



Non-skinning butyl is available without a commercial account through DK Hardware (a CR Laurence reseller). PTI brand is hit or miss there but CRL 777 is comparable to 757 and skinning, 707 is non-skinning (don't let them tell you 757 and 707 are the same, their computer is wrong). Elastiseal B100 is another decent skinning butyl, bp300 is non-skinning.

Neutral cure silicones and many other specialty sealants are available without an account through White Cap/Kenseal or, I believe, through Garvin Construction Products. All of the above will ship. Tremco Spectrem 2 is comparable to Dowsil 795 and a few bucks cheaper, other options in previous post.

any bodyshop can fix that and replace the rusted area and paint it back to original color. my 4th gen, i had to chip all the rust from the frame and painted it to prolong the life of it.

Just want to join in and say that I think the majority of this issue is dissimilar metal (galvanic) corrosion. The poor sealant probably didn't help, but that corrosion is incredibly aggressive.

Show your work.

Galvanic corrosion concentrates and can even accelerate corrosion. Notice how the corrosion is focused on the mounts on the underside of the roof? Nothing else shows corrosion, including the coated bracket. Any small amount of water that leaked inside exacerbated the issue as they likely carried electrolytes with them.

Source: background in corrosion/coatings.


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Yes, galvanic corrosion is on display here. But it's not the root cause of the issue. If it were, everyone with factory hardware and factory racks would have their roofs rusting to pieces (steel hardware has higher galvanic potential than sheet steel). But they're not.

I'm a new owner so I haven't pulled my headliner down yet, but is the stock hardware bare metal on a coated metal?

Precisely. The hardware is coated (I suspect zinc/black chromate) and the roof sheetmetal/threads are also presumably dipped, plus thread sealant. Then the roof rack itself is designed to seal off the interface from any intruding moisture. Everything is precisely designed to isolate the joint. Galvanic potential should never be realized.

@jbtvt I think I understand the difference between skinning and non-skinning, but I'm struggling to understand what you are recommending. It looks like you used skinning for the Sherpa block, but I thought non-skinning would stay soft and flexible and be a better seal. Do I have that backwards?

For a joint under compression like this you're right that non-skinning would be a better choice from a strictly waterproofing perspective. In practice I don't think it'll make a difference over the lifespan of the truck. IME butyl fails at a higher rate than silicones but usually (or always from what I can remember now top of head) due to the tendency to sag which isn't an issue here since it's basically FIPG. Never seen adhesive failure with it. So as long as the rubber spacer is set before butyl skins over I don't think it will matter, but time will tell.

My main reason for using skinning was that non-skinning always makes such a mess, so when I pull the weatherstripping out in a few years to check on things or clean the channels my roof and hands and every other thing within 300' won't be covered in butyl strings. Years of demolishing old systems with that stuff has given me PTSD. Skinning also remains pretty flexible, although not nearly as much joint movement capability as a quality silicone, it just doesn't stay as runny as the moment it came out of the tube like the non.

Unless you need to bond to rubber I would 100% recommend a neutral cure silicone either way. In old rack pic above I had 5 metal-to-metal joints at each corner of the rack - washer to washer/bolt/roof, and no leaks. And less mess than even skinning butyl, and you can reuse the extra around your windshield, or shower corners, or chimney flashing, etc

Thank you so much for that response. I'm going to be honest - That is likely one of the most informative posts I've ever seen on a forum. Learned a lot from that post!

I installed my Victory rack a couple months ago. You’re gonna love it.
I used DAP black minimal shrink silicone on mine. A RV repair guy told me it was the best for RV roofs when I had my RV so I stocked up on it for maintaining that thing. I figured it would be good for this too. So far so good. I used it to seal the holes when I ran bolts through my Tacoma’s camper shell for a roof rack. It never leaked a drop in 5 years. I do not anticipate that this one ever will either.
Lots of scary stuff on this thread. lol The key is not neglecting any part of your truck. ✌️😁