DIY - My Designs - Seam Sealing

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Seam Sealing

A fabric may be very waterproof, but as soon as you put a needle through it there are holes. These holes often get larger under tension. With the very light fabrics we are using here in some places the weave can also distort under the tension from the thread, making the holes even bigger. So some way of making the seams waterproof and strong is required. I outline two ways here: tape and seam sealer. Some aspects of how I use tape have not really been used on tents before as far as I know.


Technical comments on adhesion

"Sticky tape is sticky tape, right?" Unfortunately not. The science (some might say art) of bonding to surfaces seems to be extremely complex. What sticks to one surface won't stick very well to another. Some surfaces have a high surface energy, which usually makes them easy to bond to, while others have a low surface energy. These latter are hard to bond to. (It's all to do with van der Waals forces and dangling hydrogen bonds...) Then you have the slippery silicone surfaces, and they are in a class of their own. In fact, silicone surfaces are usually used as release surfaces for al sorts of tapes, simply because things won't stick to them. And we are trying to use silicone-impregnated fabrics.

It turns out that there are two main adhesives used for flexible tapes: rubber and acrylic. We can also use polyurethane as an adhesive, but it does not come on a tape because it cures. The rubber adhesives "bleed" around the edges a bit, and are not suitable for tents imho. Think 'Duct tape" for an example of a rubber adhesive. The acrylic adhesives are very variable: different additives 'tune' them for different applications. You can get acrylic-based tapes which make a near-permanent bond, but not from your average newsagent or supermarket: they only sell the cheapest Chinese packaging stuff which sticks very poorly. The good ones are industrial tapes, from speciality suppliers, at a slightly higher price. Polyurethane adhesives are available from bushwalking shops (eg McNett Seam Grip) and athletics shops (eg Shoe Goo), and these are very good. However, none of these bond to silnylon!

Bonding to silicone surfaces usually requires a siloxane adhesive. This is based on a silicone polymer. Tapes using this stuff are (still) hard to get and expensive, but try the web sites for major tape companies such as 3M, Husky, etc. Look for something which makes a permanent bond, not something designed to be stripped off later (eg plating tape).


Seam Sealing methods

A very simple method of sealing the seams on a tent is to erect it so the fabric is under slight tension and paint the seams with some sort of sealer. Some people use a brush, but that can spread far and wide. I have put the sealnt into a hypodermic tube and used it to run a thni bead along the stitching, then run my finger over the bead forcing the sealing into the seam. This works well.

Modern plastic hypodermics are made of polyethylene. None of the common sealants stick to this. So when you have finished, empty the hypodermic, pull the plunger out, and let it all cure to a dry state. Then peel the sealant off the hypodermic! It usually also peels off the black (nitrile?) rubber bush at the end of the plunger. This is much easier than trying to wash the stuff off - and I haven't found any simp,e successful way of washing the stuff off anyhow.

Older non-silnylon fabrics

One can buy water-based "seam sealer" in bushwalking shops for acrylic-coated and PU-coated nylon fabric, but I am not very impressed by the ones I have tried. They seem to go soggy when they get wet - which is not a lot of use. I suggest you skip them.

Polyurethane (PU) sealers such as Shoe Goo and McNett Seam Grip do seem to work fairly well on these fabrics (but not silnylon). It can be worth while thinning this stuff down with a solvent before applying it so that it can wick into the thread and holes better. However, the jury is still out on what solvent is suitable. Most solvents are fairly toxic, so be warned. I have seen suggestions that wetting the fabric with alcohol beforehand will work. Alternately, run a thin line of PU down the seam and then force it into the stitching with pressure (applied by finger!). Then you have to let it cure, which may take a day or so.

Silicone coated fabrics - silnylon

If you are using silicone-impregnated fabrics (silnylon) the PU stuff won't bond. But you can do exactly the same thing with a silicone sealer. I try to use a clear version so it does not show. Curing takes longer for the silicones.

Many hardware store cartridge tubes of 'silicone' have been found to be fairly poor as seam sealers. Some of them don't stick all that well to silnylon fabric: they peel off after a while. We don't know why. They may be good for gutters etc - for which they were designed. Also, the cartridge stuff is usually thick and needs to be diluted, but most commercially-available easy-to-obtain solvents contain some water. When you mix the silicone into the solvent the water starts to catalyse a fast curing of the silicone, so that it can go half-set while you are mixing. This is not good. Finally, those cartridges start to cure into a solid as soon as you open them. The stuff does not last. After a few months the remaining contents of the cartridge become a solid tubular block. Putting a couple of layers of alfoil over the nozzle end does extend their life a bit.

We have found that some silicone 'adhesives' are much better for our use. They typically come in small tubes and the tubes last for up to years. They are very often called 'sealants' rather than 'adhesives'. The stuff is thinner and flows better into the seams, and sticks better. You could try the following:

These have been successfully used by many people and have also been supplied by some tent companies. I have used the first two quite successfully.


The other method of sealing seams is to tape them. This has been used on parkas for a long time of course. You can get special GoreTex seam sealing tape, or a plain clear PU tape without backing. Both are normally applied with heat using an expensive machine, although you can use an ordinary iron with much care. What I have done with considerable success is to use an industrial double-sided or transfer tape, backed up with the same fabric as the tent. This will waterproof the seam of course, just as with a parka. However, by putting the tape across the seam it will also reinforce the seam. The adhesive will help transfer the tension from the fabric on one side across the stitching to the fabric on the other side without loading the thread. With the right tape you can have just a single line of stitching plus the tape. This appears to have the advantage that you do not have to leave the tent erected for a day for the seam sealer to cure, but life is not that simple.

I prefer to use a transfer tape for this rather than a double-sided tape as the mechanical properties of the carrier film in the latter affect the seam. True, the extra layer of fabric will also affect the properties of the seam, but my experience is that the transfer tape version is better. You should rub the tape down hard and then leave it for a few days for the bond to cure and develop full strength. A tape of between 15 and 20 mm width is usually sufficient for a seam, while about 9 mm width is quite enough to seal a single line of stitching without adding any strength.

What is a "transfer tape"? It is a single layer of adhesive without any carrier film. It has no strength in itself: it just transfers the load from one surface to the other. It is just the 'sticky' without the 'tape'. It's actually rather kinky stuff. The makers of spinnakers sometimes even use a transfer tape instead of stitching, because the fabric is so light that the stitches disrupt the weave of the fabric. They call it a 'seam stick tape'.

The big problem is getting suitable double-sided and transfer tapes. The double-sided tapes you buy in newsagents are not very good: you need an industrial tape. In fact, it is interesting that the industrial double-sided and transfer tapes generally have far better holding power than any single-sided tapes. The latter are made to a very competitive price for doing up packages for the short-term; the latter are used for industrial work in manufacturing. YGWYPF.

If you want to tape onto a silicone fabric you will have to use a silicone adhesive tape, not an acrylic tape (see above for why). These are not widely available, are generally very expensive, and do not bond as well as the acrylic tape bonds to ordinary fabric. Most siloxane tapes are meant to be applied and then removed later (eg for covering areass during electroplating), and do not make a 'permanent' bond. But there are also siloxane tapes which make a 'permanent' bond. The specialist adhesives companies are working on this as they have realised that there could be a market. The ones that do work must be rubbed down hard to get full contact, and they take at least 3 days to cure properly. It's actually a chemical reaction which does the bonding, not just 'something sticky'.

Alternately you can use a thin layer of silicone sealant between layers of fabric, to simulate a transfer tape. However, this is wet stuff and it too has to cure for at least 3 days. If you use one of these, check whether it is 'neutral cure' or not. Many versions release acetic acid (aka vinegar) when they cure, and this can corrode metals. There are warnings on the tubes.

A problems with all sorts of seam sealing comes when you need to (later on) sew through the adhesive. If the adhesive is not really fully cured, the stuff will stick to the thread and the needle very quickly, and cause massive snarls and break needles. Seam sealing must be done last.


© Roger Caffin 6/6/2002, 21/4/2012 30/1/2013