Make Your Own Gear: Sealing Silnylon Seams

helge—-thanks for imformation—regards barry

> 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.

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

> 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.

> 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.

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.

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?

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

Thx.

> 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

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!

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

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?).