My daughter-in-law passed this on to me recently. She was dealing with frozen and broken water lines under her mobile home in the extremely cold weather last month. My son travels in his work, and was 900 miles away, so she had to take care of the problem.
A plumber friend gave her this tip. When you get your new pieces in place, if the water is seeping through the pipe, it is hard to get the pipe to seal before the sealant is washed away from the joint.
Before you put the pipes together, form a soft ball from a slice of white bread, and push it into the pipe. Then quickly put the sealant on and put the pipes together. Make sure the ball is not too tight, and the water will wash it on through, but give it just enough time to seal the joint.
Source: Third hand tip from my daughter-in-law, Nena, as given to her by a plumber friend in Missouri.