Why does my plaster go off before I can get it on the wall?

Contaminated buckets, warm water, old stock and mixing too much are the four reasons a bag that should give you an hour gives you fifteen minutes — and only one of them is the plaster's fault.

Difficulty: intermediate
Time: 20 min
Read: 4 min

Short answer

Almost always a dirty bucket or trowel. Set plaster left on tools seeds the next mix and can halve its working time. Use two spotlessly clean buckets, cold clean water, plaster added to water rather than the reverse, and mix only what you can lay in twenty minutes while you are learning.

Gypsum plaster sets by a chemical reaction, and that reaction is seeded by existing set gypsum. That is why a professional cleans obsessively and an amateur wonders why the second bucket went off in ten minutes when the first gave them an hour. Everything else — water temperature, old stock, over-mixing — matters too, but contamination is the cause most of the time.

What you'll need

Two clean buckets
Clean cold water
A mixing paddle and drill (optional)

Step by step

Wash everything before you mix, not after.Buckets, paddle, trowel, hawk and bucket trowel. Not rinsed — scrubbed, so there is no set film anywhere. A single ring of hardened plaster round the inside of a bucket is enough to noticeably shorten the next mix.
Use two buckets and alternate.Mix in one while the other soaks clean. Never mix a fresh batch in the bucket that just held the last one.
Use cold, clean water from the tap.Warm water accelerates the set substantially. Water that has been standing in a bucket with plaster residue in it accelerates it more.
Add plaster to water, always.Water in the bucket first, plaster sifted in while stirring. Pouring water onto plaster produces lumps that never disperse and forces you to over-mix.
Mix to a smooth consistency and then stop.Prolonged high-speed mixing generates heat and breaks up the crystal structure as it starts to form, both of which shorten the working time. Mix until smooth, rest it for a minute, remix briefly, use it.
Check the bag date and how it has been stored.Gypsum plaster has a shelf life of a few months and it absorbs moisture from the air. A bag that has been in a damp garage since last year will go off unpredictably, set weak, or contain lumps that will not disperse. This is not recoverable.
Mix quantities you can actually use.Half a bucket while you are learning. A full bucket of finish plaster is more than a beginner can lay and flatten inside its working time, and the last third is always the panic.
Never re-temper.Once a mix stiffens, adding water gives you a workable-looking material that will set weak, powdery and prone to failure. Throw it out — a wasted half-bucket is cheaper than a patch that has to come off.

Tips

Keep a bucket of clean water and a brush beside you and clean the trowel every few minutes. It takes seconds and it protects the mix you are using as well as the next one.
Cold weather does the opposite and slows everything down, sometimes to the point where a finish coat will not firm up enough to trowel in an unheated room.
Retarders sold for plaster do exist, but the honest answer for a householder is smaller mixes rather than chemistry.

If it doesn't work

First mix behaved, every one after that set fast

Cause: Tools and buckets not cleaned between mixes — Fix: Full clean-down between every mix, and a second bucket in rotation.

Plaster went lumpy and would not smooth out

Cause: Water added to plaster, under-mixing, or damp stock — Fix: Correct order and technique; discard stock that has taken up moisture.

It set on the wall almost instantly on a hot day

Cause: High temperature and a dry, absorbent background pulling water out — Fix: Work cooler, control the suction of the background, mix smaller batches and shade the wall from direct sun.

It never set properly and stayed soft

Cause: Over-tempered, badly out-of-date stock, or far too much water — Fix: It has to come off. There is no treatment that recovers a plaster that failed to set.

Working time is fine but it tears when trowelled

Cause: Being worked too early, while still wet — Fix: Wait for it to firm and go matt before the flattening passes.

Questions people ask

How long should a mix last?

Manufacturers' figures vary by product, but as a working expectation, backing plasters give substantially longer than finish plasters, and both assume clean tools, cold water and moderate temperatures. Treat the figure on the bag as a best case.

Can I slow it down by adding washing-up liquid or PVA?

No. Both are common suggestions and both damage the plaster. If you need longer, mix less.

Does it matter if a little set plaster falls back into the bucket off the trowel?

Yes. That is exactly the contamination that seeds the reaction. Scrape rather than tap, and keep the return material off the fresh mix.