I've had plenty of experience with pipe repairs. In fact, when my family moved into a run-down farmhouse a few years back, I located 26 leaks in a single day ... and every one of those wet spots was a problem that required immediate attention. I think I tried about every type of patch-up job known to man ... before I came across a quick, inexpensive way to stop even the stubbornest geysers. So now, whether my plumbing springs a nagging drip or a real "Old Faithful," I just fix it with fiberglass!
In a Nutshell
Here's the basic pipe patchin' procedure:  Clean away all the dirt and rust from around the leak and plug the hole with a thick, painted—on coat of resin/ catalyst mixture.  Apply the heavy, hairy fiberglass cloth.  Coat on more liquid, and  wait for it to cure.
If your leak is in a high-pressure pipe, it'll probably be necessary to wrap the cloth all the way around the conduit, but a tiny drip under a small amount of pressure can be sealed with a patch ... as can holes in boat hulls.
The first break I ever fiberglassed was a connection between two four-inch woodstave pipes ... and that joint was wrapped in three-inch-thick concrete with screen embedded in it. The mud surrounding the area was littered with pieces of tarred burlap and strips of inner tubes from my past efforts to stop this water-waster.
It took me several whacks with a sledge hammer to expose the quarter-inch crack. Then, with my problem finally in view, I gathered together the materials: a can of resin, a tiny bottle of catalyst (hardener), some heavy fiberglass cloth (these three items can be purchased from boat stores or marine supply catalogs), a pair of scissors, a brush, and a container for mixing up my "brew".
(CAUTION: Should any catalyst get in your eyes, it will cause blindness unless immediate action—within four seconds—is taken to wash the hardener out. There is no known way to stop the destruction or to repair the damage, so—before using such a product—always don a pair of safety goggles.)
After the pipes were toweled completely dry, I pressed aluminum foil into the crack and squeezed it down tightly until the foil was flush with the surface. Then, I painted the spot with a mixture of resin and hardener ... and used enough of the chemical to assure that the wood was well-soaked.
Next, I cut a one-inch fiberglass strip (long enough to circle the crack a couple of times), swirled it about in my mixing can until it became entirely glassy, and stretched it as tightly as possible around the pipe ... being very careful not to pull the cloth apart or get it dirty. Finally, I squeezed away all the white air spots and added even more liquid compound along the edges of the patch ... to eliminate any rough spots or ridges.
After waiting a day for the repair to cure, I turned on our pump (five horsepower with a 160-foot head) and discovered that my beginner's plumbing job had worked! Only a few drops spattered down from the bottom of the pipe, the results of my applying too much liquid (when the excess dripped away, it pulled the 'glass cloth out just enough for water drops to find their way out). Fortunately, every bit of leakage eventually stopped itself, and the monster of a crack was finally dry.
That first leak, however, taught me a lesson: From then on, I always took cleanup supplies along with me. Acetone works fine to get the hardened resin off your hands, as does hot water with detergent, boraxo, or cleanser. However, if the compound isn't washed off immediately (a good idea anyway considering the strong chemicals involved), you'll spend more time trying to pick it off your hands than you did fixing the leak.
Once, I attempted to repair pipes that weren't completely dry ... a definite mistake! Even though the laminate did slow down the leaks considerably, I still had to redo my work. And fiberglass—as I quickly discovered—is harder to break apart than concrete. Hit it with a mallet, and the cloth just scrunches down. Chip it with a cold chisel, and it's like scraping an English walnut with your fingernail: The tool simply slides across the slick surface or else chops out only a tiny chunk. This substance bonds to a surface so completely that either some of it adheres in spite of attempts to break it loose, or a portion of the patched surface comes off with the fiberglass.
Naturally, the resined cloth also sticks to rust and dirt ... and when the rust flakes away, so does the fiberglass. That's why it's important to scrape, wire-brush, or sandpaper down as close to the original surface as possible.
And remember, resin never acts exactly the same ... its curing time varies according to heat. An identical mix won't set up nearly as fast in the shade or in the evening as it will at bright noon. (Humidity affects the drying process, too.)
To slow down the hardening action, add less catalyst or else repair your leaks in cool weather. I prefer, however, to add a lot of catalyst, so the material sets up really fast. Remember, too, that the compound continues to harden long after it feels solid to the touch.
Though you may have to experiment a little, try not to mix up more resin and catalyst than you can use. Too much will soon turn into a gummy consistency that quickly becomes rock-hard. In fact, my son's rock collection sports one such man-made "stone" that would have repaired two bad leaks ... if I had moved faster, or if the weather had been cooler. Now I never mix more than a cup at a time for large leaks, and much less for small ones. After all, it's reasonably easy to make more if I don't have quite enough.
When the catalyst is added to the resin, it causes a heat reaction so intense that you can't carry the container by the bottom. Since I knew the mixture could melt tar, I was hesitant to try my "miracle fix" on plastic pipe repairs. Yet when I put a patch beside a valve that had cracked the adjoining plastic and sent up a tiny spray of water, I found that only a small amount of discoloration occurred from the heat and assumed that the change in temperature helped make a tight seal.
On metal pipe in which there is gravity flow or low pressure, you don't have to be as careful to apply the liquid heavily, press the edges flat, or eliminate every air pocket. I did a half-hour's slapdash patch job on a rusty old tin pipe five years ago ... and that hundred-foot line is still completely leak-free.
That Touch of 'Glass
I can think of nothing that holds water that can't be repaired by one of the common types of resins and fiberglass cloths. Aside from woodstave, plastic, and tin, I've also used this method on steel and aluminum pipes. (The latter—which can be a problem to weld—are a snap to repair with fiberglass.)
I've also used 'glass to waterproof cement, seal the tarred edges of galvanized roofing, repair cracked windows, and take the wobble out of spigots. Furthermore, tanks coated with the right mixture become algae and rustproof ... as well as waterproof.
So, if you have any objects that leak—whether pipes, watering troughs, cement ponds (provided they've cured for at least a year), spray tanks, the block on your car, rain gutters, or valves that have frozen and cracked on the bottom—think easy and inexpensive! Think fiberglass!