Seam sealing MYOG backpack

I’d seal such a thing from the inside.  A tape made from waterproof fabrics could be glued onto the seam similarly to sewing grosgrain on the seam, it will be waterproof and it will also hide the raw edges. Strong adhesive tape (such as the tape used for cuben) could work as well and would be much easier to work with.

I looked at tape meant for cuben fiber and it says clearly in the narrative outlining its uses that it won’t stick well to woven fabrics.

I also thought about making a tape from waterproof fabrics and using seam sealer to apply it.  How would you propose to hold that in place while the sealer cures?  I think it would result in a sloppy mess.  A bunch of little paper clamps would work I guess but freshly applied seam sealer is almost like a lubricant.  Its pretty slippery.

I’ve gone full circle on this issue and now use only breathable uncoated fabrics for my packs.   I rely on plastic bags for items that must stay dry.

Uncoated bags can be thrown in the washer periodically without harming them.  That’s a big plus for a sweaty stinky guy like me.

Kelty used uncoated fabric on their original bags.

I would seal from the inside with Seam Grip, Aquaseal or equivalent urethane sealant. You’ve got two good options for constructing the seams:

1. French seams. Not the easiest way to construct a pack but french seams seal 100% with Xpac – I’m talking submersion-level waterproofness here.
2. Instead of grosgrain, bind with strips of Xpac. Not as good as the french seams but a lot easier to deal with and I haven’t had any reports of water leaking in through these seams.

We’ve made hundreds of bags with each method at Rogue Panda. No field reports of water leaking through the seams with either one, although the french seams do better when testing in the shop by filling with water (100% water retention for the french seams, very good but not perfect retention for the strip method).

Another option is the BikeBagDude method – sew the bag right side-out, so that the binding ends up on the outside of the bag. Then add any attachments like straps to those exposed seams. It’s a very unconventional look, but the inside of the pack is all very simple seams and easy to seal.

I appreciate the post and comments but most people missed the very important point that I’m a novice at sewing and I have a very basic sewing machine.  I’ve done enough sewing to know that I can’t pull off a french seam on a tube in multiple layers of stiff fabric on my sewing machine.  If I was sewing the ridge line of a tarp in a french seam it would be a different story – no problem.

The xpac strip method (although not that pretty) would work fine but I don’t see how you would hold the strips in place while the sealant cured.  I asked that question earlier today but nobody answered it.  Without stitches I doubt that they would hold up that well.  You can stitch the strips in place and then seal on top of them but in that case (other than aesthetics) why not just do a double stitch line and leave off the tape?  The tape wont add much  (if any) strength.

Of course, I could use x21rc and use cuben tape to seal the seams, but I ordered some swatches and would prefer to use vx21.  The x21rc came in limited colors and just didn’t satisfy me.  I like the white liner fabric in vx21.

The full process we do is this:

1. Sew Xpac strips over the seams (as if they are grosgrain binding – face side of Xpac on the INSIDE)
2. Trim the Xpac strips down as close to the seam line as you can get
3. Seam seal with Seam Grip or other urethane sealant

The strips of Xpac don’t add strength to the seam, but rather more water resistance. Basically they add a more labyrinthine path for water to have to take to get into the bag.

If you were to just leave the seams unbound and seal them, then water would sneak in between the layers of fabric and get into your bag very quickly. With the strips added, water has to pass into the bag, make a 180 degree turn when it hits the Xpac binding, and then has to pass through the edge of the binding where it’s covered with seam sealer. It’s not a perfect seal but in practice it works very well and I will happily stuff down sleeping bags in a bag sealed this way with no liner.

You are right that the french seam is tricky with many layers. It’s really best for simple bags with nothing sewn into the seams, like dry bags.

At some point I will write a blog post about this with pictures. There are far too many unsealed Xpac backpacks out there in the world, it’s really a fabric with unsung potential.

And I also like the VX21 better. The lining is aesthetically functional but also protects the waterproof membrane from inner abrasion. I don’t have any data on whether that actually improves the longevity of the waterproofness but I suspect it does.

Thanks again.  I design things for a living so I’ve thought this through.  The company that makes  a pack very similar to what I envision building has a video explaining how to DIY seam seal one of their bags.  They completely gloss over the area of vertical seams where the mesh pouch is sandwiched between the front and side panels.  It cant be done effectively or neatly doing what they advise at this location IMO.  That got me thinking about it.

The black fiber grid in the XPAC would probably make using xpac vx21 for the seam tape more difficult than it needs to be.  Maybe the added stiffness would make it easier to work with – dont know.  PU coated nylon would probably do the job.  What material do you think would work best for a waterproof seam tape?

Hmm. Cuben fiber would be my first thought for a transfer tape backing. I would use it before PU-coated nylon because I think using a truly waterproof fabric for the tape is better. That, or VX21RC which is softer and more flexible than VX21. Or VX03, which is even more flexible.

When using Xpac as a transfer tape backing you will want to do the opposite of what I suggested for the seam grip – folded so the face of the fabric is visible. The face has a DWR coating and will reject tape and seam grip.

I tried taffeta for the fabric on an early pack and was not happy with the results, so I’m not sure about relying on a coated fabric rather than a laminate for the tape.

I think you may have misunderstood my comment/question.  I don’t understand your answer because I don’t know what a transfer tape is (I can guess).  I was inquiring about  using the seam seal method you described but using another fabric to make the sealing strip “tape” instead of vx21.  vx21 is a little harsh and stiff and I would think anything waterproof (like PU coated nylon) would work.

I’m beginning to think that trying to seal up the seams just isn’t worth it.  The seal strip method allows moisture into the seams where it is trapped to mold, mildew and get funky.  I’m in the southeast.  Once wet stuff can take days of dry weather in direct sunlight to dry out.

Maybe a nicer design for me would be to wrap the raw seams in grosgrain and add a cuben fiber pack liner bag.  The liner wouldn’t add that much to the over all pack weight and I’m not super weight concerned anyhow.  My initial attraction to vx21 was the fact that absorbs very little water.  Its waterproof nature and the fact that the plastic film is protected within the laminate were secondary (but positive) attributes to me.

That’s a good point – the southeast is one part of the country where I haven’t lived.

I personally would never go back to pack liners, after having a truly waterproof pack. I find them to be a pain compared to just stuffing my things into the pack itself. So if I were you I might put the seams on the outside and bind them with grosgrain for breathability. But, to each his own.

Thanks for your comments.  Pretty soon I’ll have to stop thinking about it and do something.  Seams on the outside – I hadn’t thought of that one.

I was amazed the first time I hiked out west about how quickly things dry out.  On day two of my hike I turned over my ground cloth and watched the wet bottom dry out literally before my eyes.  I’d never seen anything like it.

Seam sealing with any sort of brush-on is a pain in the bum when compared to using a PET-backed, non taffeta-lined (i.e. Xpac X-series) laminated fabric and acrylic-adhesive based cuben seam tape.

My experience is that the 50d polyester taffeta liner of VX series of fabric will wear away from the PET laminate with time. The thin (.25mil) PET layer will eventually wear through as well. This has led me to believe that the thicker PET layer(.5 mil vs .25 mil) as in the X Series provides longer functionally waterproof service life(weight for weight), and is a superior materials choice for waterproof packs.

Doesnt help you much here since you’ve already used the VX fabric.

Hopefully you’ve caught the MYOG bug and will just continue to sew more packs!

Actually,  I haven’t even bought the material yet.  I did get some samples and I wasn’t sold on the x21rc.  I have no data to look at but it seems to me that the unprotected PET film even though its twice as thick wouldn’t last as long.  I also didn’t like the very limited color choice – I like colorful packs.  x21rc does dramatically improve the effectiveness and ease of getting a waterproof result.  Maybe I need to rethink it ….

 I have no data to look at but it seems to me that the unprotected PET film even though its twice as thick wouldn’t last as long.

When I read of concerns of inside durability I always wonder how and what people are packing? Everything that goes inside my pack is a soft good: quilts, hammock, clothes, food bag. There is no chance that the inside of my pack could get punctured, abraded, etc. What items are you packing that really cause concern for internal durability?

Frankly you are making this way harder than it needs to be. If you want a waterproof pack then just bite the bullet and use X21rc and tape the seams. If you want water-resistant, then use VX21 and wear a poncho or pack cover.

I’m aware of the constraints that material limitations introduce to a project since I design things for a living.  As a 45 year backpacker I’ve decided to do a MYOG backpack because nobody sells what I want.  Since I’ve been backpacking for 45 years I know exactly what I want.  I don’t want to expend the effort of making a pack to get what I don’t want – I can buy exactly what I don’t want and have it in my hands tomorrow.

I started this thread to learn enough to help me figure out how to get as close to my ideal as possible.  My motivation was to learn, not to be told “the answer” because in design there is almost always more than one answer and the best answer for a given set of constraints isn’t always obvious.

There were a few creative ideas presented that I hadn’t thought about.  Maybe I’ll use them on this project or maybe they will be used another time or maybe someone else learned something new.  Learning something new is always beneficial.

What I learned from your comments is that you really like x21rc because it meets your needs.  Your endorsement of it persuaded me to order a sample.  Your needs and mine actually differ.  I do, in fact, sometimes carry hard items in my pack that would abrade and possibly even puncture the plastic film.

You indicated that I’m overthinking it.  Again, that may be true for you.  As a design engineer I love the design process.  The creative endeavor of considering the problems and choosing the best solution makes me happy and is as much fun as using the final product.

Me too, I like to make my own stuff because I enjoy designing it.

Except I don’t do it for a living anymore, this is all I got : )

(which is much better, none of the aggravations)

Regarding your comment that MYOG is all you’ve got, I’m in the same situation.  My facebook profile lists my profession as advanced paperwork engineer.  Most engineers end up doing paperwork in the latter years of their career and I entertain myself by modifying gear I’ve bought or designing and building some of my own.  It keeps my brain engaged and makes me happy.  One of my greatest joys is repairing something that will cost hundreds to replace with some JB weld and a few dollars worth of parts from my junk bin.

I just repaired the jacket I made.  I touched the sleeve to my stove just after I turned it off.  Sliced through the jacket like butter.  McNett Seam Grip and some scrap nylon.  Keep it in the freezer and it’s good for years, otherwise it dries out in 6 months.