Black iron pipe comes coated in oil, presumably to prevent rust but possibly as a side-effect of having been machined. Does that oil need to be removed from the threads before assembly with thread compound?

My concern is that any contaminant will affect the thread compound bond.

2 Answers 2


My answer is yes threads need to be clean. However it probably depends on what is used for sealing. Black pipe is normally used for gas. I don’t want any leaks and pipe dope both gas rated and regular state the threads need to be clean and dry. I don’t remember ever seeing info on white or yellow tape but I have wiped the threads as I use oil when threading so it has become a practice to wipe fittings for gas if I thread them or not and with either tape or dope.

  • So just a dry rag wipe vs. a solvent or detergent wash?
    – isherwood
    Jan 13, 2021 at 19:01
  • I guess you could go to any level or extreme, when I worked in a clean room we had to triple wash things starting with a really nasty degreaser, then acetone, then propanol-2 (isopropyl) but that was for particulate and contamination control. I do the same wipe down not triple clean on electrical because the anti corrosion paste I use states wipe off excess oil. What is clean and what is excess ? I have done quite a few homes and never been called on this by an inspector in either trade.
    – Ed Beal
    Jan 13, 2021 at 19:13
  • Probably best practice to wipe the threads down but with residential gas pipes usually carrying no more than 2psi, I’m not too worried about leaks if you’ve taped the threads. Tape is all my plumber used (no dope) on a recent large job for me and he said the tape is really just a lubricant to help you tighten the fittings. I thought that was strange until I read another plumber mention it.
    – daneb
    Jan 14, 2021 at 5:09

Mill varnish is applied at the mill ,each mill may have it own type. Threads are cut after mill varnish is applied . If there is oil on the threads it is likely from cutting threads so cutting oil which should wipe off with rag or solvent. Mill varnish is usually more difficult to remove.

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