In article ,
Tim S writes:
coughed up some electrons that declared:

Was trying to skim the ceiling last night but it seems I've had the
bag of plaster much longer than I thought I had because it was several
months past it's use by date.

I made up one batch last night and by the time I'd cleaned the bucket
out, it had already started going off and was completely un-usable.
I haven't done any plastering for a while so thought that maybe I'd
got the mix wrong. I tried again with a much thinner mix, but again
it only lasted a few minutes before starting to go off. It was at
this point that I noticed the use-by date on the bag.

I'm just wondering what it is in plaster that causes it to go off like
that and if there's anything I could have done to keep it for longer.
The bag had never been opened, it was just one of those things where I
bought it then kept putting the job off while I did other things
instead. Obviously I put it off for too long.

Hi,

It's water from the air that does it.

That makes some of the plaster set, which you don't notice because
it's still a dust. However, you're now mixing up plaster which is
contaminated with set plaster, and as anyone who hasn't cleaned
their mixing bucket out between mixings will know, set plaster
makes new plaster set much more quickly.

I've had old plaster set as I poured it out of the mixing bucket
onto the table. You also learn this way that plaster setting is an
exothermic reaction (gives off heat), and when it sets fast, all
the heat is given off very quickly, and it gets hot.

I'm storing mine on the pallet they came on, which is on top of a tarpaulin
(damp floor) and next to a radiator.

Anything you can do to store them warm and dry helps - and scrunch up the
bag when you've used some to minimise the air that can circulate in.

But - once you're as far past the use by as you are, you're a bit doomed.
Take heart that they're only 4-5 quid a 25kg bag

As for your ceiling, I'd advise just going and buying fresh bags - it's a
hard enough job, no point in making it harder.

I had half a bag that's 4 months old - I just chucked it.

I keep a part bag which is too old. It's useful if you just want
a tiny amount, e.g. to fill in a cable chase, which will set and
polish up in 10-20 minutes instead of 3 hours.

--
Andrew Gabriel
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