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Old 05-04-2015, 10:55   #1
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Which sealant for polyethylene holding tank

Hi all,

I'm installing a new holding tank (fun!) and have to add an additional thru fitting. It's a polyethylene tank manufactured by Todd, and I'm using the Todd fitting relocation kit which includes the thru-hull (thru-tank?) and an inspection port to obtain access to the inside.

The instructions call for bedding the thru fitting with "silicone sealer" before tightening. The fitting appears to be Marelon, so what would be the best compatible sealant?

Thanks!
Gary
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Old 05-04-2015, 10:59   #2
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Re: Which sealant for polyethylene holding tank

Have you asked Todd for a more specific recommendation?
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Old 05-04-2015, 11:01   #3
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Re: Which sealant for polyethylene holding tank

Gary, How about using silicone sealant. It's all pretty much the same so pick one at your local store. Chuck
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Old 05-04-2015, 12:17   #4
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Re: Which sealant for polyethylene holding tank

Gary- AFAIK, no silicone seal is going to stick to polythene (the tank) or to nylon (Marelon is glass-filled nylon) for any length of time.


However, a bulkhead fitting like that is usually a compression fitting, with a big flange nut on the inside that is compressing the fitting where it penetrates the bulkhead (tank wall) and really, no bonding of the fitting is expected. The silicon is being used as a "form a gasket" compound. It will cure and when compressed, prevent any leaks, just like any other formed-in-place gasket. In this case, formed along the fitting and the threads, not just on the surfaces.


There are a few materials, like West Systems "G" epoxy, and some special adhesives that claim they'll bond to polythene and nylon. I suppose you could apply one of them generously and then tighten down on the fitting and let it cure, to make a permanent, fused, leakproof fitting. But just using the silicon sealer (or PermaTex Form-A-Gasket, if the instructions say it is compatible) and then just coming back the next day to snug down on the "gasket" after it has set, is probably good enough.


Whichever you use, make sure it is suitable for continuous immersion in "water". And of course, test the fitting with clean water before using the tank for real.(G)
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Old 05-04-2015, 13:38   #5
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Re: Which sealant for polyethylene holding tank

Best way to add a fitting is to take it to a plastic tank builder and have him spin one on with a router.
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Old 05-04-2015, 16:04   #6
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Re: Which sealant for polyethylene holding tank

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Originally Posted by AnchorageGuy View Post
Gary, How about using silicone sealant. It's all pretty much the same so pick one at your local store. Chuck
That's much too simple...and what would all us CF'ers do then? ;-)

You only need a sealant, not an adhesive, so I suspect any good quality silicon sealant will work. If a more exact formulation were important then Todd likely would have been more specific.
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Old 05-04-2015, 16:34   #7
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Re: Which sealant for polyethylene holding tank

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That's much too simple...and what would all us CF'ers do then? ;-)

You only need a sealant, not an adhesive, so I suspect any good quality silicon sealant will work. If a more exact formulation were important then Todd likely would have been more specific.
Yep. Asking for recommendations from strangers on the Internet after the manufacturer has already told you what to use can only result in confusion.
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Old 05-04-2015, 17:27   #8
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Re: Which sealant for polyethylene holding tank

Take a look at the Sikaflex products. They seem to have a unique sealant for any material and a very informative web site that will tell you exactly what to do. I've been very impressed with what I've used of theirs which was recommended by my yard. Expensive stuff though.


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Old 06-04-2015, 08:52   #9
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Re: Which sealant for polyethylene holding tank

Use 3M's 24 hour dry 5200.You can buy it at home depot for $8 a tube.Only comes in the color white.
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Old 06-04-2015, 08:57   #10
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Re: Which sealant for polyethylene holding tank

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Originally Posted by Gary H View Post
Hi all,

I'm installing a new holding tank (fun!) and have to add an additional thru fitting. It's a polyethylene tank manufactured by Todd, and I'm using the Todd fitting relocation kit which includes the thru-hull (thru-tank?) and an inspection port to obtain access to the inside.

The instructions call for bedding the thru fitting with "silicone sealer" before tightening. The fitting appears to be Marelon, so what would be the best compatible sealant?

Thanks!
Gary
What is the question? Use silicone. If you want a name brand, get GE RTV, they came up with silicone seal.
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Old 06-04-2015, 11:52   #11
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Re: Which sealant for polyethylene holding tank

'Probably best to ask the manufacturer--they usually have a phone number or email address to contact them with technical questions.
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Old 06-04-2015, 13:27   #12
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Re: Which sealant for polyethylene holding tank

You don't need any sealant at all, you can plastic weld with polyethylene. Use a heat gun, soldering iron (iron works better than a soldering gun, a big healthy one, that you can really lean on and a small one for stirring the joint when it's soft) and maybe a wide screw driver to smooth everything off when it's still rather pliable. You can cut a fitting out of another tank with a hole saw, and cut a hole where you want the fitting to go, hopefully you can find two hole saws that are slightly different by the dimension of the saw kerf, so that there is almost no gap to weld. If not cut the fitting larger and grind it down to fit snugly. Practice on a piece of scrap first, say a plastic oil drum. Have some extra plastic in thin strips to act as welding rod to add material when necessary. I start with the tight fit, preheat all around with the heat gun and melt it with the big iron and then stir the material together with the thin soldering iron and then smooth the joint with the big iron and the wide screw driver. It normally looks like a home made job but pay attention to detail and it works fine. The plywood pieces in the picture with the precut holes allow you to use the hole saw without a pilot drill and the center hole you don't want if you are patching where the outlet used to be. Clamp the plywood where you want the hole and it keeps the hole saw in position. I used the green poly plastic because it was already like welding rod.
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Old 06-04-2015, 14:41   #13
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Re: Which sealant for polyethylene holding tank

I've used UNISEALS. Dirt cheap and no sealant required.
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Old 06-04-2015, 16:09   #14
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Re: Which sealant for polyethylene holding tank

Guys....the OP has a kit from Todd which includes all parts necessary to install a new fitting. It works much like a thru hull fitting. No adhesive, welding, etc necessary...just a small amount of silicon sealant.
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Old 06-04-2015, 16:16   #15
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Re: Which sealant for polyethylene holding tank

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Originally Posted by belizesailor View Post
Guys....the OP has a kit from Todd which includes all parts necessary to install a new fitting. It works much like a thru hull fitting. No adhesive, welding, etc necessary...just a small amount of silicon sealant.
I was wondering what the question was. A through hull, tank, and some silicone, job done.
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