Craig Powell, technical manager at Kelmore, explains why priming is so often a necessity, despite the debates!
You have probably heard or even used the phrase “it’s a crime not to prime”. This is one of several priming sayings doing the rounds and although some might feel this statement, when thought about in its literal sense, is a little strong, it emphasises the importance of priming backgrounds prior to tiling or levelling.
Priming is something that has seemingly always been done, and more than likely quickly completed without too much thought. For some tilers, this process will be undertaken as soon as possible after arriving on site, probably applied and left to dry whilst the rest of the tools are unloaded from the van and set up ready to start. Due to it not being a time-consuming process, both in its application and the waiting time for it to dry, priming can be completed habitually in the knowledge that it is the correct thing to do without questioning the finer details of its importance (pretty much in the same way that I accept and try to take on-board my doctor’s advice to lose weight just to be healthier without the need to probe further).
Priming has numerous functions; to name but a few, aiding adhesion on non-porous backgrounds, creating a barrier to eliminate adverse reactions between the background and subsequently applied adhesives or levelling and smoothing compounds, and the one I am going to elaborate more on, controlling the porosity of backgrounds. Controlling porosity of the background is very important when using cementitious products. A common misunderstanding is that cement gains strength from drying, but this isn’t the case. Cement gains strength whilst hydrating, and therefore if a cementitious product prematurely loses too much water because of the background’s porosity, then the product will not achieve its optimum strength.
Priming can also sometimes be said to give the background its first drink or to stop the background from being thirsty. Both phrases describe how, by correctly priming a porous background, the cementitious product can be allowed to fully hydrate and gain strength, whilst also ensuring a longer open time of the product. Where levelling and smoothing compounds are concerned, in addition to allowing sufficient hydration of the product, priming will also allow the compound to flow better and maintain a longer wet edge.
Priming will ultimately result in a better surface finish, one with no or fewer pinholes. Pinholes are a common side effect of insufficiently primed backgrounds. These are created when air from the background is pushed to the compound’s surface, creating a bubble, and due to the product’s diminished open time, when the bubble bursts the compound is unable to flow and smooth over the hole created. There is no doubting that cementitious products will bond to unprimed porous backgrounds. Just remembering how many times we have all had to scrape unwanted material, like plaster drops and splatters, from a cementitious screed highlights the fact that porous backgrounds will draw moisture from wet building materials and the suction will create a bond.
Obviously, adhesion to a background is a fundamental requirement, but it needs to be strong and long-lasting. Adhesion can – but should not be – achieved at the expense of a product’s integrity, performance, longevity, and in the case of a flooring compound, its surface finish. Now I must state that it isn’t always necessary to prime, and depending on the specific background and product being used it maybe suggested not to do so. However, there is no denying that for the vast majority of everyday applications priming will yield a better result, or even stop a potential failure.
I feel this was summed up quite nicely by a quote I remember from a debate by two tilers. In response to being told that there was no need to prime a certain background, there was the swift reply of: “Yes, we also don’t have to go to work, but it’s better if we do!”