I remain a fan of silnylon shelters. Cuben remains excessively expensive, for me, and aside from higher levels of waterproofing it is hard to get excited about PU coatings, due to longevity issues. Silnylon stretches and can be made waterproof for a very long time, with the former being a still underestimated benefit in severe weather, and the later our subject for today.
Not too many years ago the waterproof-ness of higher end (sil and cuben) shelter fabrics was a matter of much discussion. Despite protests to the contrary, the conservative side won out and all the top echelon shelter makers today advertise higher hydrostatic (water entry pressure) figures. 3000mm is a good number for holistic warm and fuzzies, but older fabrics, as well as those subject to much UV exposure or abuse, can easily have numbers in the triple digits. 700mm gets the job done 90+% of the time for backpackers outside places like the coastal rainforests of the world, but misting (minute precip drops blowing through your shelter) does happen, something I can now better appreciate, having been through storms like the one on the west coast of New Zealand where it rained for 12 hours in a manner which seemed to beat much of the available oxygen out of the air. The technical side of the discussion involves size and impact angle of raindrops, and it is generally sufficient to know that your shelter will deal with all probable conditions, especially because it is so easy today to have exactly that.
My orange 5’x9′ tarp has seen a lot of action in the past year, as it’s a nice shelter for the desert, where large storms are usually predictable, and has been put to use as a ground cloth for both sleeping (when we forgot our tyvek) and laying out meat when field dressing game. The probability of a few small nicks in the fabric, plus the slightly low stock HH, had me wanting to recoat it before the spring season got going.
For recoating a shelter I use the same technique I do for seam sealing, but make a more dilute mixture, and use a 4+” wide foam brush. You’ll go through a lot of mixture here; fitting the above juice bottle 3″ deep was only just enough for this small tarp.
Once your silicone is dissolved completely, paint it on. I coat the whole surface with brush strokes one direction, then go back for a second coat at 90 degrees. Unless outside temps are very hot it takes several hours for the coating to cure completely, making dry patches (which bespeak of less coating) easy to see and address. Once finished the surface should look slick and glossy. Allow this to cure until totally dry, which on a sunny but cool (55F) day could take 5-6 hours.
Obviously, doing this on a bigger ‘mid will not be the fastest project.
In conclusion, it is worth discussing the virtues of silnylon. It remains the most easily compactible of shelter fabrics (this tarp stuffs down to the size of an orange), and the only one which can be easily maintained in the manner here discussed. The stretch of sil also makes it very strong in use, damping the impact on stake points, which is by far the most common way for tarps and mids to fail in the field. Sag overnight, especially after heavy rain, is the common complaint, and one which cannot be wholely done away with. Nylon stretches when it gets wet. What can mitigate sag, to a great extent, is very aggressive pitch tension, which takes advantage of the stretch inherent in sil, as well as the strength which comes from good reinforcement and panel orientation.
Good sil, like the above, properly reinforced in a way which avoids point stress along stitch lines, also like the above, will take an impressive amount of force. Plastic linelocs will first slip, and then when tied off conventionally, simply snap long before the fabric rips. When the sil does eventually rip it will tell you a lot about the quality of your reinforcements. Laminated ones like these that use a similar fabric with similar amounts of stretch avoid point stress. Heavier reinforcements with less stretch will tend to rip at the edge, due to uneven transfer of load. Stitch lines tend to exacerbate this, though the force required to rip any decent tieout is high enough that distinctions here are largely academic.
Equally, if not more important, is how the panels of a shelter are layed out. Silnylon stretches far more diagonally (on the bias), than square with either direction (the warp or weft). Making a seam bias to bias has the potential to create more potential for stretching than the shelter shape can handle. This is why the corners of the BD Megalight can never be made truly tight, and why that shelter deflects in the wind more than it really should. MLD also has bias-bias corners, but they use a stronger and less stretchy fabric, as well as panel with no middle seam, and can thus be tensioned more aggressively and completely, making the signature MLD hum in high winds possible.