Send any questions or comments to the designer Roger Caffin
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.
"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. 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. They are industrial tapes. Polyurethane adhesives are available from bushwalking shops (McNett Seam Grip) and athletics shops (Shoe Goo), and these are very good.
Bonding to silicone surfaces usually requires a siloxane adhesive. This is based on a silicone polymer. Tapes using this stuff are hard to get and expensive. Alternately you can use a silicone sealer such as the clear window and fishtank stuff available in hardware shops. However, this is wet and has to cure for at least 24 hours. 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. If you want to thin it down you can try ordinary turps. Other solvents may also work, but you need to check. You can get silicone solvents, but at a price.
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. One can buy water-based "seam sealer" in bushwalking shops, 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. On the other hand, polyurethane sealers such as Shoe Goo and Seam Grip do seem to work fairly well. 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.
If you are using silicone-impregnated fabrics the PU stuff won't bond. But you can do exactly the same thing with silicone sealer. I try to use a clear version so it does not show, and I try to use a non-acid-cure version for safety. You can get these silicone sealers quite cheaply in the hardware shops, and McNett have a (dearer) silicone version of their sealer as well (which I have not tried). You would have to go to a bushwalking shop for the McNett one. I have also used a special Dow Corning 3140 RTV silicone of low viscosity with some success. Curing takes longer for the silicones.
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.
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 are using a silicone fabric you will have to use a silicone tape, not an acrylic tape. 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, and do not make a 'permanent' bond. But the siloxane area is very new, and may improve with time. The specialist adhesives companies are working on this as they have realised that there could be a market. The ones that work must be rubbed down hard and take at least 3 days to cure.
© Roger Caffin 6/6/2002