"Bob F" wrote in message
...
On 8/1/2020 5:37 PM, TimR wrote:
On Saturday, August 1, 2020 at 7:24:57 PM UTC-4, %% wrote:

Probably fumes from the trap and drain system.

Unlikely to be fumes if the stain is where liquid would run.

Put some water down it and flush it out and it may help.


WTF do you think is happening in that drain?

Most likely something like emptying the bath
is producing a surge up the drain which sees
some dirty water from the drain coming out
the overflow and its that the stains the sink.

You wash down all sort of caustic and organic materials

Its less clear that that is happening
much with that infrequently used sink.


Key word -- infrequently
Dirty water sits and rots.

But doesnt move out the overflow down to the drain.

The stain is exactly where liquids would flow and shaped like they would
be.

But it's hard to see a backup happening out the overflow that didn't also
show at the drain.

I thought of fumes, but they would have to be heavier than air.

I scrubbed it off this morning, too late for a photo. If it recurs I'll
do that. This is the sink I use to brush my teeth, I don't think wife or
daughters ever use it, certainly not on a regular basis. So it gets
about one drinking glass of water per day, but that's every day.

On the other hand. About ten years ago I replaced the faucets in that
sink. I don't remember why but I wouldn't have done that job without a
darned good reason. I do remember the old ones didn't want to come out,
I think I beat on them with a rubber hammer for a long time before they
moved. I had to pull the sink from the vanity into the backyard to get
it apart.

So it's not impossible I screwed something up when I replaced it. But it
hasn't backed up or dripped underneath, so I can't have done too badly.


If you have the bowl filled with water and suddenly open the drain,

He said that it has never had a plug in it

the pressure of the water at the drain will surge water into the overflow
drain tube, perhaps enough to splash some out through the drain hole or
above it where it can then run down and out.