This is a frequently reposted set of tips on sealing seams of tarps and tents
made from silicone impregnated ripstop nylon. Two published sources exist. The
first is Ray Jardine's Beyond Backpacking. The other is Ed Speer's Hammock
Camping. Each one has a section on the construction of silnylon tarps.
This is what I do:
Buy a $3 tube of clear silicone seal from the hardware store. It is used to seal tile joints around bath tubs. I put about a teaspoon of this in a jar lid, and then add about two tablespoons of solvent. I mix this with a small stick and then a small paint brush. Then I use the paint brush to paint a half inch stripe along the seam. The silicone soaks into the small spaces and saturates the stitches. I have not had any leaking with this method with several tarps. The silicone is not thick enough to appear as a separate layer on the tarp. It has not peeled when applied this thin.
Jardine recommends paint thinner as the solvent. Speer recommends white gas (Coleman type) as the solvent. I have used both, and white gas seems to disolve the silicone a little easier and dries quicker.
Note: this is for silnylon only. I do not recommend it for PU treated nylon. Seam sealer of the standard variety is better for that purpose.