Plaster and render mixes are designed to have the optimum working and set time, allowing each layer to be applied and finished before starting to dry in readiness for the next layer or the final set. These times are usually ideal for working with, but there are some occasions when you need extra time to finish a set, or you need it to go off a bit quicker. The size of the area, the weather, and the skill level of the plasterer can all affect the timings; and in these cases you can use additives to slow down or speed up the drying times of the product to suit.
In the summer, plaster and render retarders are useful for keeping the mix workable, when the extra heat and humidity causes it to set more quickly than it should. Extratime plaster retarder allows between thirty minutes and one hour extra time to complete and set. This is good for ceilings and large walls that would otherwise be a challenge when hot weather sets finishing plaster too quickly. It works by slowing down the crystal formation that sets gypsum plaster solid and comes in sachets portioned for a single bag.
Silomix retarder is designed for bricklaying and plastering mortars, and while the average plasterer won’t have much call for bricklaying products, the additive would be useful for anyone working on buildings with very uneven walls to plaster. By having a slower set time on levelling mortar a better, smoother finish can be achieved that will be suitable for the finishing plaster to be applied to. The product gives up to 24 hours retardation, which means that on large uneven walls a very level finish can be created over the space of several hours, taking the pressure off the plasterer.
Evermix 3 in 1 is a render additive that waterproofs, plasticizes and increases setting times for external wall coverings. It is useful for large outside areas that are being finished in hot weather, as it ensures the render dries slowly enough to adhere to the wall properly and will not crack or blow soon after drying. The extra properties aid in the slower drying and superior finish of the finished wall surface.
Plaster retarders are also useful for amateur plasterers who are working on an area larger than they are used to. The extra time allows for some reworking of each coat or application over a large surface area, which may pose a challenge for the inexperienced plasterer. Getting the hang of plaster setting times is incredibly useful for novice plasterers who intend to make a career of it; so apprentices and those in training, may find it more appropriate not to use retarders when starting out, saving them for when they have a better intuitive feel for setting times but need a little extra help when the weather is warm.
In the winter plaster and render accelerators are more useful, as the mix will dry more slowly than in the summer anyway. They are especially useful for external rendering, as it is imperative that any application of render to a wall in the winter will have to stand up to the worst of the weather, which can not only affect the drying times but the integrity of the surface as well. Many accelerators also act as a frost proofer, stopping the water in the mix from freezing and causing cracks when it fully dries.
GoRend Speed Up is a new accelerator on the market which allows two coats of render to be applied and set in the same day, removing the problem of low overnight temperatures affecting the setting. By using a product like this there is no need to worry about protecting a setting surface overnight with sacking and tarpaulin. Like Extratime, it comes pre-packaged in the right amounts for one standard bag of render. It works at temperatures above freezing, so will not work well when temperatures are persistently below zero degrees.
When the ambient temperature is very low, Everbuild’s frostproofer and accelerator is your best bet, as it works in sub-zero temperatures. This is a liquid product so requires accurate measurement for it to work properly but it will protect against frost damage while the mix sets, and speeds up the setting and drying times in very cold weather as well as in normal conditions, when a more rapid set is needed.
Baumit Speedtop is an accelerator designed for silicone and acrylic renders rather than cement and gypsum based ones. It is also a liquid product bottled in the right amounts for 25kg of render and also works to speed up setting times when humidity is high; it works in conditions up to 95% humidity.
As with retarding products, these accelerators can be difficult for novice plasterers to use, as the setting times change dramatically from the norm. While a retarding product simply slows the set times down, an accelerator speeds it up significantly, and if the person applying the plaster is not quick enough it can set far too soon, leaving unfinished layers that need to be removed before starting again. Before using one of these products it is a good idea to get feedback from other plasterers in your area, who are dealing with the same climatic issues, and if possible wait until the weather is warmer to do the job without the challenges of cold and rain.