What to do with out of date plaster? (2026 Update)

I've a couple of bags of multi finish around 3 years old. The plasterer we occasionally use won't use them.

I'm trying to think of ways to get use from them.

Does it set hard enough to use to level a floor for example?

It will set hard, but you will then have a very lumpy, non-level floor :-)

If you have a problem with clay soil, you can dig it into the soil to improve it. It's gypsum remember. Someone else did it and I used a bag of old bonding plaster to mix into my back garden before sowing grass seed.

It did in my bathroom, under tiles, when I ran out of self-levelling.

Owain

Not surprising really - old plaster sets *fast* so no chance of getting a bag of it on the wall in time.

Well in smaller quantities it might be useful as a general filler, so any place you might normally use polyfiller or similar.

Might be ok if it were being boarded over for example (say under a laminate or engineered wood floor). It will crack away at the edges though if feathered out, and walked on more directly.

I buy price-reduced out-of-date plaster when on offer fot just that.

If it's 3 years old it'll be rock hard before he's even finished mixing it! (no exaggeration either)

I use it for filling chases, but not to the top. Very handy because it sets so fast, and it cracks badly giving a good key for your finishing layer. If it's not bonding coat, adding a little PVA will make it bond better. Plaster is not easy to use as a surface filler anyway, so you'd do better to top off a chase flush with something like polyfilla.

You can only mix in small quantities because of the speed it sets. Also, a plasterer told me he accidentally mixed up a bucket of old plaster and it set so fast it was steaming away and melted the bucket.

If you paint the chase with a 50:50 water:PVA and let it dry and before filling the chase wet down with a another dose of water/PVA you should find no, or limited cracking. Holes created by chases or for electrical back boxes have a large surface area with respect to the amount of plaster you are going to use to fill it and if the hole is into porous material water will be quickly sucked out of the infill plaster causing it to crack. Very old plaster can set without cracking,

I too found old plaster great for positioning electrical back boxes into solid brick walls. Setting in 5 minutes gives just enough time to position a box.

I currently have two bags of fairly fresh plaster that will be used in the next few weeks and any remnants will be lightly spread on my lawn as I have done in the past.

I keep an OOD bag of Wickes fine surface plaster for that purpose. Unlike gypsum plaster it can be mechanically scraped flat with a steel ruler if it sets too quickly. Also sandable.

yes. Sorry, typo. Wickes Fine Surface filler. Used to come in two sizes both smaller than the noraml 10kg plaster bags, so take up less space in storage.

Even fresh, it can be made to set more quickly by adding some plaster of paris. Convenient for fixing plaster edge stops and filling around electrical back boxes etc.

Yup. Same thing happens with car body filler. They're both noticeably exothermic reactions.

Tri Sodium Citrate is the favoured way to slow down the setting of plaster. You might have to do some testing to get the mix right.

Gypsum is the hardened state of plaster of Paris calcium sulfate hemihydrate which is essentially crushed and dehydrated gypsum. The reason it has a use by date is because even if you seal it in a plastic bag moisture condensates. It will naturally soak moisture from the air. This means that even if the bags are in date, say you keep them in the house (lots of moisture in the air) the crystals will begin to slowly hydrate. Unless the bag is new, the bags will set faster and faster with age. It's no joke either, some bags can literally go off in the bucket before you pull out the paddle. Once the calcium sulfate has set out long enough say a few years what you find is that most of the crystals have formed, but not bonded so when you add water some bits might go off, but most of it will never set and create a failed surface, 0 uniformity. Technically you can just stick the plaster in the oven and dehydrate it, but not worth the cost and it does require proper heating. Toss it in the trash, recycle it or bury it, once it has hydrated into gypsum it is inert, but don't put it with organic waste unless you are sure it has fully hydrated, because certain bacteria will convert the sulfate into hydrogen sulfide which smells like a number 2 after Guinness and eggs.

From May 15, 2020.