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I am trying to find out what is coating the outside of the 3/4 copper domestic cold water supply pipe in my basement. The house was built around 1950. The length of copper pipe that runs from an ell after the meter up to the floor joists above has been coated with some kind of very thick gunk, about 1/8" thick, possibly some kind of cementitious paint or just plain paint. A lot of it appears to have flaked off, and there are also some short sections of the pipe that do not have this gunk anywhere on them. Was it common practice to coat that supply pipe from the mains, and if so what was the purpose?

coated pipe1 coated pipe2

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  • maybe it is accidental
    – jsotola
    Oct 27, 2021 at 17:28
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    Is the back of the pipe (facing the wall) also coated? If not, it was probably just a sloppy job at coating the wall.
    – jay613
    Oct 27, 2021 at 17:29
  • @jay613: The rear side is not entirely free of it but it has far less, so I think you may be right. In the late 1980s, previous owners had the basement remediated after removal of rigid asbestos insulation from around the hydronic heating pipes, and that included painting the walls to encapsulate any asbestos fibers.
    – mr blint
    Oct 27, 2021 at 17:56
  • probably keeps it from sweating as much on humid days by insulating the metal some. Not common afaik, but not a bad idea if it helps.
    – dandavis
    Oct 27, 2021 at 20:14

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From your photos it looks like the same material as the walls. Some sort of masonry paint is my best guess.

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