Things You'll Need
2 pipe wrenches
Iron pipe thread die (optional)
Pipe joint compound
Galvanized iron cap
Galvanized pipe was popular as water supply plumbing for many years because of its strength and durability. Eventually, corrosion takes its toll, narrowing the inside diameter and slowing water to a trickle. Replacement of the pipe usually takes place in phases, which means at some point a section of plumbing must be closed off while a new one is installed. Closing, or capping, a section of galvanized pipe can be either a temporary or permanent measure.
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Turn off the water. Disassemble the run of pipe you want to close off, working backward from the last faucet or other fixture on the line to a threaded connection at the capping point. Use two pipe wrenches when disassembling each joint to avoid disturbing any joints in the remaining plumbing.
Remove all traces of rust and old pipe joint compound using a stiff wire brush. Wipe the threads clean with a rag. Inspect the threads for damage. Repair lightly damaged threads by running them with a die of the correct size for the pipe, which is also called "chasing" the threads.
Apply joint compound liberally to the pipe threads as well as to the threads of the cap. Thread the cap onto the pipe and tighten by hand.
Use two pipe wrenches to tighten the cap on the pipe. Hold the pipe being capped securely with the wrench. Be careful not to disturb the other joints in the remaining plumbing run or you may cause a leak.
Turn on the water and check for leaks.
If the threads of the pipe you're trying to cap are damaged and chasing them with a die doesn't help, try using a waterproof epoxy instead of joint compound. If the cap still leaks, the pipe will have to be cut and threaded.
Although it's possible to weld iron pipe, don't attempt to weld galvanized iron pipe without prior experience. The galvanized coating burns with toxic fumes and must be removed first.