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Ideally one would use the joint compound with the desired setting time, together with the recommended amount of water for mixing. However, a few times I added more water when mixing products like SheetRock Easy Sand 5 Lightweight Setting-Type Joint Compound to gain longer working time, the result turned out fine. I added enough water until it is almost runny. The mix probably remained usable for 20-30 mins. I'm curious what may be the downsides to this practice.

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    If you need more working timing don’t use setting type compounds. They are harder to work and harder to sand than standard mud. watering it down compromises it’s chemistry, sheetrock mud non setting you can water it down to your happy but not setting mixes. I recommend rookies not use setting compound because of all the horrid messes I have found over the years. The one exception is around a sink or tub. Setting compound is water resistant compared to regular but then make small batches and always cold water, (hot water speeds the reaction and shortens the working time) – Ed Beal Dec 3 '20 at 22:10
  • @EdBeal Yes, but sometimes one just has/wishes to work with what's available, hence the question :) However, for applications the finishing type compounds will do, what disadvantages are there to weakening a little bit the strength of the setting type compounds? – Roc W. Dec 3 '20 at 22:21
  • There are no real downsides if you are using drywall mud for patching drywall aside from overdoing it and getting runny/useless compound. – TylerH Dec 4 '20 at 15:34
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Adding water to prolong working time is known as "retempering". In general it's not a good idea as it dilutes the concentration of cement components in the mix. This means that the resulting material is more porous. It's often specifically prohibited with things like tile mortar, where strength is a critical concern.

In the case of joint compound it's not so serious a problem. However, unless you mix vigorously you'll start to see lumps in the mix as it sets up. Your best bet is to mix small batches, use it within the stated (or actual) working time, and toss what's left. Use cool water to slow the curing reaction. Warm water dramatically shortens working time.

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    Definitely cold water one of the worst jobs I ever saw was a house the owner thought you were supposed to use hot water, you could see every patch through the front window at 30 miles an hour it was so bad we just pulled all the Sheetrock.+ – Ed Beal Dec 3 '20 at 22:13
  • More porous so it's probably less water resistant and weaker, but once the solvent water dries, will it be worse off than drying joint compound? Since one would typically prime & paint afterwards, I assume the practical difference is very little? – Roc W. Dec 3 '20 at 22:42
  • That's basically what I'm saying. – isherwood Dec 4 '20 at 13:57
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    The critical chemical difference with setting or 'hot' muds is that the water does not simply dry or evaporate, like with joint compound, but rather takes part in, and is consumed by, the chemical reaction that bonds the compound together. By diluting it with water you end up with something that doesn't set together like hot mud should, but rather turns into a wet slurry with excess water that needs to dry like joint compound and isn't as strong. – J... Dec 4 '20 at 14:26
  • Wouldn't the water over the stoichiometry simply evaporate, just like excess water in cement/concrete does? The setting-type joint compound has many of the same ingredients as cement, albeit in certainly different proportions. Workability and morphology of the cured product can be an issue, but I wouldn't think chemistry is? – Roc W. Dec 4 '20 at 22:21
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Most of the compounds I've used usually say you can a little more or less water depending on what the application is for. Like Stove Top Stuffing: add a little more water for more moist stuffing or less for dryer stuffing. If you add too much water to your compounds, then you'll end up with compound soup and it just won't stick to the wall.

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Cement is kind-of "the first epoxy". It's 2 separate ingredients coming together to create a third: namely portland cement and water.

Epoxies have 2 things going on: Drying, and curing. These are separate processes. They are not related, and each one runs on its own timetable.

So if someone is "a little bit clever", they might find a way to keep it workable (un-dry) despite the fact that the curing process is running its pace. The problem is that the point of curing is to create chemical bonds, and you are breaking those bonds when you continue to work it. Thus, you will have fewer bonds holding it together, and the material will come out weaker.

Such materials cure on sort of a "half-life": Half the cures happen in X time, and half the remaining cures happen in the next X time, etc. etc. So the curing is "front-end loaded", and disrupting the cure early takes out most of the strength.

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  • Sounds like it's mostly weaker strengths? But at least it's still stronger than the non-setting type? – Roc W. Dec 3 '20 at 22:24
  • They make strength different ways, so it's hard to say. – Harper - Reinstate Monica Dec 4 '20 at 1:26
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As long as it wasn't too much water it will not cause any issues. It's commonly done by professional drywallers all the time.

I'm not sure what the limit is but I'd say if the compound becomes runny that's too much.

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