After having read the MYOG article on seam sealing (the one in which it describes thinning the silicone with mineral spirits before application), I still have one simple question.

Does one need to seal both sides of the seam (I'm talking about the ridgeline of a 11x10 silnylon cat tarp), or will the top seam be sufficient?

Many thanks.

The top should be sufficient. Make sure to setup your tarp before sealing. Silnylon should be taut when you seal it. Otherwise the silicone layer could tear when setting it up if it was not evenly applied. It isn't a big deal. A dab of Silnet will patch it... if you catch it before it rains!

It wouldn't hurt to do both sides. A gram of prevention is worth a litre of water. Something like that :)

I may seal the underside at a later date for extra measure, but tomorrow is a crunch day. I'm going out on a short hike to test all my new UL gear, and sealing/drying both sides is likely 1 task too many for tomorrow.

Thanks for the replies