"TimR" wrote in message
...
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.

Thats the crucial detail that needs to be explained.

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

That might well be due to how the overflow comes into
the drain before where the plug goes in the drain hole.

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

And wouldnt produce a stain that looks like liquid
running down. The stain would be much wider and
less defined even if the fumes were 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.

Since you never put the plug in and never fill it, I'd
block the overflow with something and see the stain
never show up again and then block it permanently
but so that the block can be removed if you sell the
place or start using it as a filled sink again later etc.

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.

The usual reason is because you cant stop it dripping
and cant work out how to replace the washer seat etc.

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.

Yeah, they can seize quite easily with some poor
designs with no way to get enough leverage.

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.

Yeah, maybe so a backup ends up with enough
backing up the overflow to be enough to stain
the sink with that dirty backup.

But it hasn't backed up or dripped underneath,
so I can't have done too badly.

But might have ended up with some sealant etc
that you used to stop it dripping redirecting a
backup up the overflow accidentally.