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I'm trying to replaster the walls in an old house in Taiwan. Due to both the extreme humidity here and the shorter drying/curing times, I would prefer to use setting-type plaster over drying-type.

I think I've found these products and their names in Chinese (石膏 and 批土) – it's tough to be sure with the language barrier and geographic differences in industry, but they're both made of gypsum, and one comes in big sacks of powder while the other comes in pre-mixed buckets (~US$10 for 50lb and 5gal, respectively). However, everyone I've talked to here suggests either using drying-type by itself, or mixing the two for heavier repairs. Nothing I've read on any American forum suggests this mixing method, and nobody in Taiwan advocates using the gypsum powder all by itself.

I tried their way – deep patches are still wet (hard but cold) the following day. I tried using just the powder, mixed with water – it goes from pancake batter to unworkable, crumbly mess in the span of 15 minutes. From everything I've read, working times on setting-type plaster should be 70-90 minutes.

What's going on here, and what can I do to get a longer working time but keep a strong, moisture-resistant plaster job? With my total lack of experience, waiting 36+ hours between patch jobs and coats is really slowing the project down.

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The working time of setting-type plaster is a consequence of that plaster's ingredients. In the US, setting-type drywall compound (e.g. Sheetrock Easy Sand) can be had in a wide range of drying times from 5 to 210 minutes. If your mix is getting crumbly after 15 minutes, you might just have the wrong product.

Combining setting and pre-mixed compound is not something I've seen recommended. Final dry time, strength, shrinkage, etc. will be hard to predict. Perhaps you can work out something that does what you need this way, but I think you'd be better off finding a compound that gives you adequate working time when following manufacturer instructions. You can also find plaster retarder and accelerator that should let you adjust the setting time.

Alternatively, you can try to make do with what you've got by mixing setting-type compound and quickly filling the big cracks, and then top-coat with the pre-mixed or drying-type stuff. That will let you cut out a bunch of the waiting, but then fine-tune with a slower-drying plaster.

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