Make Your Own Gear: Sealing Silnylon Seams

[Benjamin Smith]

Companion forum thread to:

Make Your Own Gear: Sealing Silnylon Seams

[barry hitchcock]

helge—-thanks for imformation—regards barry

[David Corbin]

Can I assume correctly that McNett SeamGrip thinned with mineral spirts will work similarly well for nylon or PU cloth tents? And will Husky 2500 tape also work on them?

[Roger Caffin]

> Can I assume correctly that McNett SeamGrip thinned with mineral spirts will work similarly well for nylon or PU cloth tents? And will Husky 2500 tape also work on them?

I am not sure whether SeamGrip thins all that well with mineral spirits – you would have to try it out with whatever passes for mineral spirits where you live. I believe it thins very well with Xylene, but try buying that at your local hardware store!

Husky 2500 tape will NOT bond well to PU or nylon. It uses a siloxane adhesive which reacts chemically with silicone coatings to make a really good bond, but it bonds poorly with non-silicone surfaces. On the other hand, a sailmakers seamstick tape (eg 3M 9845 I think) does bond very well to PU.

[Vick Hines]

Just read Jay Ham's article on thinning silnylon, and have a few observations.

Discussions with the technical folks at Duco (the glue and sealant people) produced the following information:

1. Thinning silicone resin with mineral spirits or anything else weakens the cured resin because the slurry structure interferes with the resin forming long polimer strings as it cures.
2. If you want thinner silicone sealant, buy a thinner silicone sealant. It will be a unitary formulation containing no thinners, just a thinner formula of resin.

"Permatex Flowable 100% Silicone Windshield Sealant", available at any auto parts store, is such a thin, unitary resin.

I have used Permatex for both sealing and glueing seams for over a year and just finished 6 months on the AT with all my silnylon seams sealed with it. Compared to Silnet, it is tougher, less sticky, more liquid with better penetration, and perhaps more UV resistant. It cures to the touch in 30 minutes, sets as a glue in 20 minutes, fully cures in 24 hours. It's also cheap – half the cost of Silnet for twice the amount of product.

[Roger Caffin]

Hi Vick

Thanks very much for posting this information. A few comments:

> 1. Thinning silicone resin with mineral spirits or anything else weakens the cured resin because the slurry structure interferes with the resin forming long polimer strings as it cures.
Well… that depends on the solvent.
If you use a volatile solvent which can evaporate quickly, it should not interfere very much. But such a solvent may be more difficult to buy.
If you use a silicone oil (eg Dow Corning OS-2 I think) as the solvent it will integrate with the silicone polymer and should not affect it at all. But buying silicone oil is even more tricky.

> Permatex Flowable 100% Silicone Windshield Sealant", available at any auto parts store
I am off and googling!

**Edited Dec-2008: not cheap, but it worked very well. Recommended.

But note that some thin silicones meant for other tasks do not like to be used in thin layers. I have one such silicone, very liquid, but incapable of curing in typical thin seam-sealing applications unless I brush catalyst over the silicone a day later. Painful… It was meant for fine detail casting.

Cheers

[Bill B]

I know the conventional wisdom is to seal the under side of a tent fly or tarp rather than the outside. However, if you don't care that the outside of the fly shows seam sealer, wouldn't you get a more waterproof job by sealing the outside? It seems to me that by sealing the underside only, you allow water to work into the seam before it is stopped by the sealer. Why not stop the water before it has a chance to get into the seam?

[Roger Caffin]

> if you don't care that the outside of the fly shows seam sealer, wouldn't you get a more waterproof job by sealing the outside? It seems to me that by sealing the underside only, you allow water to work into the seam before it is stopped by the sealer. Why not stop the water before it has a chance to get into the seam?

Since the edges of the seams are usually on the inside of the tent rather than the outside, I agree 100%. After all, my tent is not there as a fashion item: it is there to shelter me. I don't care if the seams show a band of sealant. And it takes a lot less sealant to treat the lines of stitching, compared with trying to cover the whole seam.

[Frank Ramos]

A much easier way to seam-seal is this. Put a dab of seam sealer on a piece of paper. Dip the brush in this dab. Spread the sealer on the seam. After about 30 seconds, the sealer will absorb moisture from the atmosphere and swell up. Avoid smearing the sealer after it has swelled up since this weakens it. This process is very simple. After you have finished, wipe the brush off on the paper so you can use it again later and then throw the paper away.

If you thin the sealer, it will be weaker, and yet one of the main purposes of sealer (at least for silnylon) is to strengthen the seam, since the line of needle holes becomes a natural line of weakness in the fabric. You don't want the sealer to make a globby mess (and it won't if you first put it on paper and then transfer it to the fabric like I am suggesting) but at the same time you DO want the sealer to be visibly thick enough so that it is obviously adding strength to the seam.

It helps to have high relative humidity in the air while the sealer is curing, such as by closing the windows and running a hot shower before applying the sealer. This is mainly an issue when sealing indoors in the winter, when the relative humidity indoors is sometimes extremely low. I don't worry too much about the fumes when sealing indoors since I'm retired and hence don't need my brain anymore. But if you are worried, then just leave the house for a few hours after you finish sealing with a window cracked and all the fumes should be gone when you return.

[Roger Caffin]

> It helps to have high relative humidity in the air while the sealer is curing, such as by closing the windows and running a hot shower before applying the sealer. This is mainly an issue when sealing indoors in the winter, when the relative humidity indoors is sometimes extremely low.
Good point. Here in Sydney, Australia, the humidity is usually well above 60%.

> I don't worry too much about the fumes when sealing indoors since I'm retired and hence don't need my brain anymore.
Cackle!
But in fact silicone sealant does not emit harmful fumes. Some versions emit a slight smell of vinegar, while others don't: that's all.

[F. Thomas Matica]

I have used the silicone/mineralspirits solution to recoat a pair of old REI overmitts. The original coating had dried an pealed off on the inside of the packcloth portions of the mitts. I have read that it is not possible to recoat when this happens. Well, I gave it a try anyway and it seems to work just fine. The packcloth has regaind its waterproof character. A really thinned out coat seems best as it is less likely to crack or peal off. Is it as durable? No matter, just a simple task to recoat. the thinned out silicone really seeps into the cloth where the original coating had worn off. I'm thinking of using this method on my day pack or at least portions of it….as another experiment.

[Deb dePeyster]

I couldn't find the GE Silicone II clear sealer that everyone recommends for silnylon. I looked in 3 stores. What I did find was the above.

Can I use it the same way, mixed with mineral spirits?

It comes in a caulk tube or a small squeeze tube.

[Roger Caffin]

I have used the Permatex Flowable Windshield sealant on my latest winter tent. I did not bother to thin it. Instead I applied it with a 20 cc hypo to the seam and rubbed it in with my finger. It dried fast and seems to have done a very good job.

I have used un-thinned hardware store clear silicone wet-area sealants with a hypo as well. A little thick, but they worked fine. So I don't bother with thinning very much these days.

As an aside: I find the hardware store sealant makes better non-slip stripes on my groundsheet than the Permatex. A detail.

[David Adler]

Unfortunately i read the article on sealing silnylon seams after i sealed my BD Lighthouse tent in conventional manner.
My question is, has anyone used the mineral spirits thinning technique with regular Seam Grip?

[René Enguehard]

I'm glad someone finally addressed the whole inside versus outside seam sealing question. Previously I have heard people say that an inside seam seal is better because it doesn't degrade as fast as the outside one since it isn't out in the elements. That being said, it stands to reason than an outside seal would prevent water from coming in completely, thus providing a more efficient seal.

Good to hear a definitive answer on this. :)

[E C]

Any further insights on diluting McNett Seam Grip with paint thinner or rubbing alcohol to flow better?

Thx.

[Open Space]

Do the mineral spirits degrade the fabric properties?

[Roger Caffin]

> people say that an inside seam seal is better because it doesn't degrade as fast as the outside one since it isn't out in the elements.

That may apply to PU sealing – not sure.
But it does NOT apply to silicone-sealing silnylon. For a start, there is not a huge amount of difference between the silicone polymer coating on the fabric and the silicone sealant.
Secondly, check out typical Tile and Gutter sealants: these are meant for use very much on the outside!

Cheers

[Shawn Peyton]

Just joined for the sewing tips but found this article. Thanks a lot! This was great to share and made my seam sealing chore today a lot more smooth. Cheers!

[Gary Dunckel]

Roger, can we thin McNett's Seam Grip Seam Sealer with mineral spirits, as folks do with silicone-based sealants, or is there a chemistry issue that precludes the success of this?

I want to seal the seams of my Marmot Helium EQ bag, like Tony Wong did (and recently reported on in the forums). Would it be better to use McNett's Sil-Net (silicone), or their Seam Grip seam sealer (polyurethane)? Or do they both do the job on non-sil-based materials (properly bonding to the shell fabric, longevity, durability when stuffing into a compression sack etc.)? For minimal mess, proper penetration into the seams, and the lightest finished weight, I think I should thin the sealant material first. Do you have some advice for doing this? Since it is an expensive bag, I need to do it right the first time. Thanks for your valued help!

[Ken Thompson]

Permatex flowable windshield silicone. Comes with an applicator tube tip, does not need to be thinned. Thinner already than Mcnetts sil sealer.

[Roger Caffin]

Hi Gary

Yes, you can thin PU adhesives and sealants down, but what the best solvents are is something I haven't researched much. I am fairly sure solvents such as Xylene would work fine, but this class of stuff is not normally available outside a chem lab. Instead I have used pressure to force the PU sealant into the surface.

Can you use a silicone sealant? I wouldn't recommend it because I am fairly sure the SB fabric will have a fluorocarbon DWR on the outer surface. Fluorocarbons and silicones don't like each other: they do not bond together. Incidentally, this is also why you should never try to use a silicone spray on a DWR-treated jacket.

A possible alternative would be to try using a good DWR (such as the Atsko fluorocarbon one) along the seams. Not absolutley waterproof, but still pretty good.

Cheers

[Gary Dunckel]

Thanks so much for the quick reply, Roger. I wondered about possible chemical issues among these products. So Seam Grip it is. I'll try mineral spirits as a thinner, since I already have some, and test a seam first to see how it works. We all truly appreciate the science you bring to the BPL table, sir. All the best!

[Gary Dunckel]

Roger, if I decide to try the Aktso fluorocarbon DWR, can I later do the McNett polyurethane seam seal technique? Or does the fluorocarbon DWR prevent close molecular bonding of the sealant to the microfiber shell material and seam threads? The shell already has a DWR, as you surmised, but I'm not sure what type (but a good company like Marmot would probably use a fluorocarbon DWR, wouldn't you think?).

[Mark Berry]

Note for anyone outside of the States or Canada – mineral spirit is called white spirit everywhere else. It took me ages to find that out, but it's the same stuff.

Mark

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