Today we called a plumber to repair a leak in the seal of a toilet drain pipe. He used metal-filled epoxy putty to seal up the area around the leaking joint, as actually disassembling the joint looks to be a major project due to where it is located. This was his second repair in the same spot in three months.

The plumber asked for regular (Canola) cooking oil, which he kneaded into the epoxy. This looks suspect to me, as I've always believed that one should remove all oil, dirt, grease, etc. from areas to be epoxied.

Why add oil to the epoxy?

  • 1
    Sheer speculation, but I believe most epoxy putty sticks are soluable in oil so a tiny amount of oil would 'thin' it. This would certainly make it easier to knead it together thoroughly (and more importantly make sure the hardener was mixed in evenly). I'm going to have to try this...
    – Comintern
    Sep 4, 2015 at 23:23
  • I agree. Also, if the epoxy hadn't been used for awhile and not sealed ,even though the two parts hadn't been mixed they still might have become "stale" or dried out. The oil made it softer as they were kneaded together.
    – ojait
    Sep 5, 2015 at 3:35
  • This wasn't to repair a cast iron pipe was it? Was he trying to repair the oakum and lead connection like they used in the old days? Stuff oakum and tamp pour molten lead on top for a seal.
    – ojait
    Sep 5, 2015 at 3:39
  • This is for a plastic pipe, but I'm not sure what type of plastic. When I asked the plumber himself why he mixes in the oil, he said that it helps prevent the epoxy from sticking to his fingers. Though, he seems to be more the type to "do as he's seen" and not "understand what he is doing" so I don't know if that really is the reason for the practice.
    – dotancohen
    Sep 5, 2015 at 6:48

1 Answer 1


I recently learned something about injection molding. While this is a completely different process, it may be somehow related to this topic.

When plastics engineers want to produce a flexible or partially flexible part, they choose to make the parts from elastomeres in the past (e.g. rubber or silicone). Unfortunately elastomeres are nearly impossible to produce during an injection molding process. Thus flexible parts were rather expensive.

Some smart engineers tried to mix cooking oil into thermoplastic materials like polyethylene and polypropylene to make it softer. They eventually found out, that the outcome was more like an elastomere, but could be produced by injection molding, which makes it rather cheap. In fact, if you encounter a cheap plastic part like a toothbrush these days which exhibits a flexible part, it is very likely that the flexible part is basicly the same polymer as the adjacent parts but with cooking oil mixed in.

And because cured epoxy has some thermoplastic properties, I think it may be possible to increase the flexibility of the cured resin by thoroughly mixing in some oil. And flexibiliy is the be-all and end-all of a sealing.

This leaves the question, why oil on a surface is detrimental to the adhesive strength of epoxy, and inside the compound it's not.

I guess it depends on the degree of dispersion. One will have to mix it really good to prevent the formation of an oil film preventing the epoxy to grip the parts.

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