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Taming the Beast: Extending Plaster Setting Times for a Flawless Finish

Do you ever need to slow down plaster setting times? Ever mixed up a perfect batch of plaster, only to find it setting before you could achieve that smooth, flawless finish on a large area? The struggle is real, my fellow plastering professionals. Thankfully, there are ways to extend plaster setting times when working on very large walls and ceilings.

In this blog, we'll delve into the world of slowing down plaster setting times and explore the best methods to achieve a stress-free, well-finished project:

The Science Behind Extending Setting Times:

Gypsum plaster has been around for a very long time, and the way it's made is pretty straightforward. It all starts with gypsum rock, which is ground into a fine powder and heated to high temperatures in a process called calcination. This heating process removes some of the water from the gypsum, transforming it into hemihydrate or calcined gypsum. When the hemihydrate is subsequently mixed with water, it creates a thick plaster mixture that can be spread thinly and set hard. This hardening happens because the hemihydrate absorbs the water and changes back into gypsum, forming tiny needle-like crystals that lock together as it sets. The final resulting product is a strong, versatile and breathable plaster.

By introducing certain additives when hydrating the hemihydrate we can tweak this setting process, buying ourselves valuable minutes (or even an hour) to work our magic without affecting the strength or other properties of the finished plaster.

Commercial Options: The Plaster Retarder's Arsenal

  • Ready-made Plaster Retarders: These are your best bet for ease of use. Available at most builders merchants, they come in powder form and simply need to be mixed in according to the manufacturer's instructions. Make sure you use a product that is tested to BS EN 13279-2 : 2014 and recommended by City & Guilds. Popular brands such as Eazymix Ltd.’s Extratime Plaster Retarder offer extended working times of up to an hour, are fully tested and accredited by City & Guilds. 

The DIY Approach: Tread Carefully!

While tempting, experimenting with household ingredients like cream of tartar is not recommended.

With cream of tatar and other experimental additives the results can be unpredictable. As extensive testing has not been carried out on these additives it is not known how they might affect the plasters strength or finish.

Beyond Additives: Mastering Your Mix

Here are some additional tips to get the most out of your plaster's working time:

  • Mix Only What You Need: Don't be tempted to go overboard with the batch size. A smaller amount means less plaster to work through before it sets.

  • Cool It Down: Hotter temperatures accelerate setting times. Use cool water when mixing, and avoid working in direct sunlight. Try not to work in temperatures above 25 degrees and when working in warm temperatures windows should be kept shut to avoid the warm air drying the face of the plaster and causing crazing. If crazing does occur, or the plaster is drying too quickly, work should be stopped and reattempted when temperatures drop. Refer the the plasterer manufacturers guidelines before plastering if in doubt.

  • Work Smarter: Practice makes perfect! The more comfortable you are with your technique, the quicker and smoother your application will be.

Remember: Safety first! Always wear gloves, eye protection, and a dust mask when working with plaster.

By implementing these strategies, you can transform plaster from a time-sensitive foe into a cooperative ally. With a little planning and the right approach, you'll be plastering those large ceilings stress free!

Browse our range of plaster additives and scrim here or contact us today for more information.



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