Seam Sealing: A Thread for Tips and Questions

Discussion in 'DIY & MYOG' started by WilliamC, Sep 4, 2020.

  1. WilliamC

    WilliamC Thru Hiker

    There is quite a lot of information about seam sealing on TL but it's scatted around a number of threads. As I have a question and searching around to see if it had already been answered took a lot of time, I thought it would be useful to have a thread specifically for advice on the topic.
    Clare, Diddi, Teepee and 1 other person like this.
  2. WilliamC

    WilliamC Thru Hiker

    To kick us off, I have always seam sealed flysheets on the outside as I figured it would stop moisture getting into the seam, where it might be very slow to try out. I recently picked up a tube of Silnet (actually Seam Grip + Sil) and notice that the instructions say to seam the inside of lap felled seams.
    Other than aesthetics and perhaps prevention of UV damage, can anybody think why it might be better to seam the inside rather than the outside?
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  3. Fossil Bluff

    Fossil Bluff Section Hiker

    Myles21 and WilliamC like this.
  4. WilliamC

    WilliamC Thru Hiker

    One thing that is often is misunderstood about silicone sealers is about how they "dry". In fact, silicone cures on reacting with moisture, so you want a humid day, rather than a dry one. Warmth helps, too. Take your tent to a sauna with you :)
    Rmr likes this.
  5. WilliamC

    WilliamC Thru Hiker

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  6. SteG

    SteG Ultralighter

    I personally went with @Mole suggestion on a previous thread and weighted the apex with a heavy kettle bell, stretched it and applied straight out of the tube and used my finger to rub it in. Ive done this on two shelters now and worked a treat both times. No drips and looks ok.
    Ps just did the outside and got in the grooves as well. If covered properly not sure why you would need to do the inside too. A bit of a tedious job to have to do inside also.
    WilliamC likes this.
  7. Teepee

    Teepee Thru Hiker

    Silicone sealant is often more UV resistant than the fabric and protects the stitches too. Also easier to apply. It's got an easier job though on the inside as the seam stops a lot of water ingress.

    It also helps stick the threads and makes the seam stronger.

    Do both sides I say. :)

    Flowable windshield silicone sealant does the best job on sil tents and stuff sacks etc. It flows into threads and gaps, sets smoother and has a much higher modulus when cured than a lot of sealants.

    For the strongest bond and longest lasting seam sealing on PU coated and non siliconized substrates, PU sealant like McNett seamgrip is the best.

    To thin Silicone, you can use white spirit/white gas/lighter fluid effectively.
    To thin PU, Xylene paint thinners is very good.
  8. WilliamC

    WilliamC Thru Hiker

    To be clear, the Silnet instructions are to do ONLY the inside.
    SteG likes this.
  9. SteG

    SteG Ultralighter

    Oh, I'm not sure what I used now. It's what ever MLD send with their shelters William
  10. Rmr

    Rmr Section Hiker

    It will dry sauna or later.
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  11. OwenM

    OwenM Section Hiker

    Ffs, I'm going camping in the wilds not going to an art show. Splat it on with a finger and get on with your life.
    cathyjc likes this.
  12. SteG

    SteG Ultralighter

    Just familiarising myself with MLD sealing instructions and it says in bold text to only do the outside when using sil net??
  13. Stuart

    Stuart Ultralighter

    Perfect timing @WilliamC !!

    I need to seal the ridge of our car camping tent, a Quechua 4 person job. It's in the style of a Nallo GT. The seams of the flysheet are taped but over the years these have come away slightly in places. To be clear the tape as a whole is intact, there are just a few spots where it's come away. I'm not sure if I'm best removing the whole tape and sealing it all or just trying to fill the few places where water is getting in.
  14. WilliamC

    WilliamC Thru Hiker

    My only experience is with a couple of sil/PU nylon waterproofs, which had been taped on the inside/PU side. I left the tape in place and sealed the outside with silnet. The tape continued to peel.
    I suspect that if you only seal the gaps, the tape will continue to peel in places and you'll get new leaks.
  15. Fossil Bluff

    Fossil Bluff Section Hiker

    Yes, there's a reason for that , on that shelter... bu99ered if I can remember why, it depends on the type of seam. Thats how I did my Hex - works a treat and looks neat and tidy - but there was a good reason to do the outside. I'll have to search again.
  16. WilliamC

    WilliamC Thru Hiker

    The instructions say inside for lap felled seams, outside for butted seams.
    upload_2020-9-4_17-15-0.png
    In the video, it looks as if it has both types of seam, though I thought butted seams weren't recommended for flysheets as all the stress is on one row of stitches.
    I'm going to go with the outside, anyway. If it's good enough for Ron Bell...
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  17. Fossil Bluff

    Fossil Bluff Section Hiker

    Here we go, this was from Bob's website where the Youtube is lifted from.

    Seal the Seams - Most of seams on these shelters are designed using butt seams (See below). These seams keep the the threads on the inside where they are less exposed to dirt and grim. Butt seams must be sealed from the outside of the canopy. Applying sealer to the underside of the seam will not seal them.
    WilliamC likes this.
  18. Myles21

    Myles21 Ultralighter

    I haven't heard of sealing a sil shelter on the inside. I also like Bob's (BPL UK) seam sealing video - the Hex Peak that he's sealing is only sil coated on the outside (PU on inner) so he applies it to the exterior. Both he and Franco (on YouTube) advise using a 'splodge' of sealant and mixing the white spirit in gradually to get a 'thick olive oil' consistency, no need to measure proportions. I've found that I need to add a splash more white spirit a couple of times during the sealing process, as the mix thickens due to evaporation. I also found that the sealant can thicken in the brush so it helps to have a little pot of white spirit on hand to keep the bristles unclogged.
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  19. Myles21

    Myles21 Ultralighter

    Is this any help?
    https://www.msrgear.com/blog/tents-101-seam-taping-vs-seam-sealing/
  20. Fossil Bluff

    Fossil Bluff Section Hiker

    I followed his instructions WRT thick olive oil. I found the process quite satisfying, I thought it would be a nightmare :D
    Last edited: Sep 4, 2020
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  21. Myles21

    Myles21 Ultralighter

    I know, quite absorbing and meditative.
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  22. WilliamC

    WilliamC Thru Hiker

    Well, I seam sealed the Aricxi Mid today and I think I know the answer to my question.
    If you seam on the exterior (the upper side in the diagram below), its hard to get the the silicone under the roll (at x) without making a mess. If water gets in at x, it can easily get through the stitching at y. However, if you seal on the interior the stitching at y is protected and it's relatively difficult for water to get through to z. So, it seems to me that seling the interior would be more likely to be watertight.
    Lap felled seam.JPG
    I also found, seam sealing with temperatures in the upper 30s, that sweat beads up nicely on the tents nylon - hopefully a good sign for its waterproofness :D
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  23. WilliamC

    WilliamC Thru Hiker

  24. Enzo

    Enzo Thru Hiker

    I've got a lap felled folder coming for my twin needle sewing machine.
    No idea why.

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