Soldering is the traditional method for joining two or more metal objects together. It’s a two-part process, the first being to clean the material, normally with a mild acid; and the second is the use of a heat source to melt liquid metal into the joint.
The material used by Your Local Plumber is a 50/50 solder – 50% lead and 50% tin.
This joint is incredibly strong, durable and completely water proof; lasting well over 100 years. After the joint is soldered it should be cleaned again and then painted, assisting with waterproofing.
Generally soldering is best on unpainted and new materials. While Your Local Plumber are experts, it is always hard to clean old instances to a high enough level and to then re-solder joints, especially if old solder is present.