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Old 07-30-2021, 06:28 PM   #1
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Silicone or Polyurethane?

I will be attaching my L brackets to my bus roof with 1/2" bolts straight through the hat channel. Question is what is a better roof sealant: poly or silicone? I plan on covering the whole assembly with Dicor. And then painting with Henry's. I just cant decide what to squirt into the holes and under the L brackets. No fiberglass on a bus roof everything is steel.

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Old 07-30-2021, 07:23 PM   #2
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Originally Posted by Chrise View Post
I will be attaching my L brackets to my bus roof with 1/2" bolts straight through the hat channel. Question is what is a better roof sealant: poly or silicone? I plan on covering the whole assembly with Dicor. And then painting with Henry's. I just cant decide what to squirt into the holes and under the L brackets. No fiberglass on a bus roof everything is steel.


There is always a better choice than silicone except maybe for building aquariums. I give tubes of the stuff to my worst enemies for their birthdays.
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Old 07-30-2021, 08:28 PM   #3
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Wow, thanks for your quick reply. I so enjoy hearing from passionate people about whatever, and it seems you have a passionate hate for silicone. Excellent! But why?And also you would go with Polyurethane, or something else?
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Old 07-30-2021, 08:35 PM   #4
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Sikaflex 295-UV
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Old 07-30-2021, 08:50 PM   #5
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^^ Danjo, that Sikaflex 295 is good stuff.



For the use described, anything is better than silicone.

Urthane based products are preferable. Some use asphalt or even acrylic. I used Dynatron 550 on each rivet of my roof patches. Same, on the flange bolts, of the rear deck a/c.

Are you using washers or flange bolts outside?Completely though the hat channel with nut/washer inside? What size are the holes in the L bracket?
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Old 07-30-2021, 09:30 PM   #6
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Both the Sikaflex and the Dynatron referred to above are great and reliable. I'm also really impressed with the 3M 5200, which is also a moisture-cured polyurethane adhesive/sealant.
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Old 07-30-2021, 10:40 PM   #7
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Quote:
Originally Posted by Chrise View Post
Wow, thanks for your quick reply. I so enjoy hearing from passionate people about whatever, and it seems you have a passionate hate for silicone. Excellent! But why?And also you would go with Polyurethane, or something else?

I'm glad others have replied with some answers more helpful that my anti-silicone rant.


Silicone caulk is a pet peeve because it fails always. Removing every trace so a better choice can replace it is usually difficult and sometimes impossible. I've spent summers removing silicone from windows where it failed. Spent hours and hours removing it from floors, bathrooms, woodwork... you name it. Oh how I dislike silicone caulk. Silicone caulk fails any where it's used. But it sells like hot cakes... Comes in sooooo mannnny colors toooo! OH I HATE THAT STUFF!!!!!!!



Oh my, I need to take my relaxer now.....


hehehe just playing and not really mad
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Old 07-31-2021, 01:14 AM   #8
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Anything that is NOT silicone is the way to go.
As everyone has said sikaflex, 3M products and urathane’s are the way to go.
Please reconsider using any silicone, It is really not a good adhesive and it can not be painted. Bed all your roof vents, hatches and penetrations with something like sikaflex then cover in Dicor is a great plan. Follow that up with yearly inspections on your Dicor and you’re good to go.
Also Henry’s is silicone based.
Good luck
Cheers
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Old 07-31-2021, 01:21 AM   #9
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Excellent help! Thanks!

Wow I love this site for just that reason, so many helpful peeps. I will steer completely clear from silicone and also find another elastomeric roof coating that is not silicone based.

Cheers!

Chris
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Old 07-31-2021, 07:07 AM   #10
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3M 5200 is a very (very) strong adhesive but only for **permanent** use cases

much too strong if you ever want to undo a join without destroying the substrate.

4200 is just as good a sealant but easier to remove when needed.

And yes the Henry products are silicone, excellent reputation for that final coating seal

but I don't know how we don't hear about the usual horror stories, it must at least adhere to itself in subsequent applications years later
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Old 07-31-2021, 01:49 PM   #11
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Well my plan is to use 1/2" bolts up through the hat channel and the skin, then through the L bracket which is made to accept a 1/2" bolt (but there is some wiggle room), perhaps a washer if there is clearance for it, then a nylock nut. Any comments or help would be appreciated. BTW this whole rack will be constructed of off the shelf uni strut components.
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Old 07-31-2021, 04:07 PM   #12
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if you are looking at unistrut stuff then look into spring nut that fit and lock into the unistrut itself.
then the only thing anchored through the roof is the strut itself.
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Old 08-01-2021, 12:25 AM   #13
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Stuff like solar panels, anything with high lifting force at highway speeds,

do not rely on anything but the most robust connecting hardware to hold it down
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Old 08-01-2021, 06:53 AM   #14
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Quote:
Originally Posted by Chrise View Post
Well my plan is to use 1/2" bolts up through the hat channel and the skin, then through the L bracket which is made to accept a 1/2" bolt (but there is some wiggle room), perhaps a washer if there is clearance for it, then a nylock nut. Any comments or help would be appreciated. BTW this whole rack will be constructed of off the shelf uni strut components.
Are you using strut or just the other strut parts? Hereís what I did:

I mounted the Z-brackets on the panels. This gave me the center-center measurement for the strut.

I used some masons twine to find the center of the roof and used that to space out the strut.

I drilled each end hole and secured that and then did the ones between.

For the attachment points i used

Carriage bolt from the inside. The low profile head is unobtrusive. (I think 1/2Ē is overkill, but itís fine)

Sika 295-UV around the hole

Neoprene-backed fender washer went against the roof surface (does the neoprene add anything the sika doesnít? I donít know)

The unistrut

A double-thick fender washer

A nylock nut

Hereís a pic of the panel attachment. Thatís a strut nut covered by a saddle washer
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Old 08-01-2021, 08:19 AM   #15
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looks good

a flex washer might help maintain a gap for the sealant

metal to metal torqued down squeezes it all out
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Old 08-04-2021, 03:47 PM   #16
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We used 3/8 Rivet Nuts. We drilled holes in hockey pucks, sealed those down with a urethane based sealer,, and attached uni-strut with 3/8 bolts thru hockey pucks into riv- nuts inside hat channel. Saw this on Be Adventure Partners. Being a Yankee and a Redwinga fan, I loved it. Had to send away for pucks cuz I now live in the south, but we have been very happy with this configuration. Plus it raises the unisteut an inch which wasperfect for our solar panel mounts. We used 1.5 inch angle iron for everything else up top.
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Old 08-04-2021, 06:01 PM   #17
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I agree that silicone isnt good for this application. And fyi; cured silicone wont stick to cured silicone, and nothing much else will either.


If you're having problems with silicone sticking then the substrate hasnt been cleaned properly.
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Old 08-04-2021, 07:28 PM   #18
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This is fascinating! Y'all have some great recommendations for sealing roof holes. Do these same products (3M 5200, 4200, Sika 295-UV...) work well for sealing windows? The silicon I put on them years ago has long failed, as you knew it would, and I was planning on using more of the same. .. but something that lasts longer would be nice.
Thank you!
(I'd be lost without y'all and skoolie.net!)
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Old 08-04-2021, 07:39 PM   #19
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image.jpeg
Urethane for sealing your windows
Just a few bucks a tube at Home Depot
The Sika Flex would be good here also.
Good luck
Cheers
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Old 08-04-2021, 09:21 PM   #20
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Quote:
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There is always a better choice than silicone except maybe for building aquariums. I give tubes of the stuff to my worst enemies for their birthdays.



and your still alive
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