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SEAMS 4 - Restoring seam water-repellency using silicone emulsion

Here is an effective cure for leaking seams on an old tent.

If the tent's outer skin is made from SIL/SIL-finished fabric (silicone elastomer on both faces of the fabric and no seam tape) then the leakage may be due to the fabric finish or the water-repellent treatment used on the thread being degraded due to age or wear and tear.

If the tent's outer skin in made from a PU-coated fabric and PU seam tape - usually clear - has been used to factory seal the seams on the inside face, the leakage may be due to delamination of the tape (possibly from exposure to extreme temperatures), or chemical or UV degradation of the tape. In this case refer also to the page on 'heat-setting' old PU seam tape. The thread used to stitch seams that will be tape-sealed in production is not of the same 'anti-wicking' quality as that which should be chosen for SIL/SIL fabric seams.

The preparations for and the application of the silicone emulsion are almost identical to those described on the DIY page COATING 5: The SIL/SIL coating is leaking .

The ingredients and tools required are a tube of CLEAR Silastic® RTV (or other high-quality silicone rubber sealant), say a litre of mineral turpentine and a 12mm wide paint brush - all quite cheap and obtainable from just about any hardware store. Don't choose the cheapest brush. It will loose bristles while you work, leading to unnecessary frustration.

Clean the outer tent thoroughly to remove any dirt or dust. In this case concentrate on the seams themselves. Use warm water, detergent and a soft brush along the seams - then hose off thoroughly. Do this with the tent up in the direct sun, pitched tautly. After ensuring all traces of detergent are washed off let it dry out completely - nylon fabrics take longer than you might expect. Unlike the case where the entire fabric surface is to be coated with emulsion there is no need to move the dry tent into the shade before going to work on treating the seams.

Mix one part silicone sealant with three parts turps in a container with a screw lid. 100ml of mixture is enough to treat the outer tent seams on a typical backpacking tent. A 250ml clean glass food jar is ideal. Shake the jar until the silicone and turps have emulsified to a lump-free liquid. The consistency should be like a thick plastic paint, easily worked but not 'runny'. Add more turps as required, and once more shake until an even consistency is obtained.

The aim of the exercise is to saturate the seam stitching, just the thread, with silicone emulsion, getting at every leakage pathway water could take. Think in terms of gumming up the thread and needle holes so they become so highly water-repellent and blocked that leakage becomes impossible. The emulsion will also saturate the fabric yarns where it is painted on but there is no need to leave an unsightly and pointless excess of silicone elastomer on the fabric surface. This will be wiped off.

Apply the silicone emulsion with the narrow brush just along the seams but also working the brush bristles under the folded edge of the seam fabric. It is much easier to work on a tautly pitched tent than a slack one. Work on about a one metre length of seam at a time. Use a clean white cloth to remove excess emulsion from the fabric surface. The need for lots of pieces of cloth can be avoided by first removing the excess with toilet tissue or kitchen paper, discarding that, and then finishing with the cloth to wipe off the inevitable small flecks of tissue fibres that will adhere to the wet surface.

Where there is an insertion into a seam, like a reinforced guy attachment point, treat both sides of the insertion. Capillary action will draw water in between two fabric layers that lie against one another - unless they are made highly water-repellent with the silicone.

There is no need to work on the vertical seams low down on the outer tent skin. It really doesn't matter whether these leak or not, they are not going to drip onto the inner skin or onto the tent floor. Any leakage will just run down the inside to the ground.

Once the job is finished leave the tent up for six hours to allow the turps to completely evaporate and the silicone to cure.

Regarding the paint brush, it is probably best to just discard it since the quantity of turps required to thoroughly rinse out the silicone so the brush can be re-used makes the attempt uneconomical!
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