A product that does what it says!
ATSKO Silicon Water-Guard Sealant
I found mine over at Wally World for under six dollars. A quick bit of Internet research on the hammock and tent backpacking forums had persuaded me that plain old ScotchGuard was basically useless in this situation, but there seemed to be a lot of people who were recommending this ATSKO product.
I gave the old tarp a brief cycle through the washing machine with some gentle, “non-detergent” detergent, let it dry completely strung up out in the sun, and gave it a thorough treatment that used up one full can on a 9′ x 11′ tarp. The instructions include the idea that you need to shake the can up regularly while you spray. I found that a steady circular motion not only gave me very efficient coverage but also kept the little ball bearings rolling around almost constantly. There was almost no fluid left in the can when the propellant was exhausted. Just like they claim. The fluid went on smoothly to penetrate the nylon taffeta fabric very evenly, and it was also easy to determine how evenly the spray was covering the surface without any signs of soaking or drips. I didn’t have any doubt that I had followed the instructions and gotten the results that were indicated. This was yesterday morning, and this morning the heavy dew-fall had beaded up almost microscopically on the whole surface. A couple of quick flip/shakes and the whole tarp shed all the moisture. … You can’t ask much more than that.
Six dollars to renovate a 40-year-old tarp seems like a fairly good deal. Back in the 1970s PU treated nylon had just started to be used, and this was a pretty expensive tarp back then, so I’m grateful I was able to bring it back to full usefulness. We will string it up this weekend for a little shelter up by the river, and I’ll let you know whether it works out as well as it seems to.