board finish - shelf life.

[kdband]

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.

[Tim S]

[e-mail] coughed up some electrons that declared:

Hi,

It's water from the air that does it.

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.

Cheers

Tim

[Lobster]

Bzzt! Unlike most prepackaged foodstuffs (cue new subthtread?!) plaster's definitely something not to use much beyond its sell-by date. The behaviour you describe is absolutely par for the course I'm afraid.

David

[Andrew Gabriel]

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 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.

[kdband]

Thanks for the responses guys. Lesson learnt.

Bought a brand new bag yesterday and it went on like a dream.

Maybe this is why have found it difficult sometimes in the past, I've never noticed the use-by date before. I usually look at the job upfront and try and buy everything I anticipate needing for the job. Plastering is one of those jobs I don't really like doing so I often put it for as long as I can. In future I'll only buy the plaster when I'm ready to use it.