I'd been using foam pads for backpacking for a long time, but last summer I bought a used Thermarest through Craigslist. What an improvement. But after I used it continuously for 3 months in the Drakensberg, the Outback, and the Fiordlands, it would only hold air long enough for me to fall asleep. It would slowly deflate and somewhere around 1am I'd finally settle down to the ground, get a rock in the hip and have wake up to reinflate the pad. No good.
Got to patch dem leaks!
Step 1: Find the Holes
There are a couple ways to do this. I used my bathtub, which is probably fastest. Fill it with about 3 inches of water and then submerge sections of the pad until you find where the bubbles come up. Have a sharpie nearby so you can dab the pad dry and mark the hole. You can also sponge the pad with some soapy water to find the leaks.
Once you've marked all the leaks (mine had two), set it out in the sun to dry.
Step 2: Plug the Holes
I used some polyurethane epoxy to patch the holes because it has a bit of flexibility even after it is fully cured. I had some nice Double-Bubble pouches lying around. Regular epoxy would probably work just fine too as long as you don't coat too big of an area as the epoxy might pop off. If I were really clever, I'd find a way to fix this Thermarest in the backcountry.
I opened the valve to deflate the Thermarest before applying the epoxy so that the air wouldn't blow a path through the curing epoxy.
Mix it up on a piece of cardboard pulled from the recycling bin and dab it over the place you marked with the sharpie.
Let it set for a few hours and then test out how well it holds air!