Description #
In this episode we will be covering all things surface porosity. What it is, the effects different levels of porosity have, and appropriate substrate preparation for different levels of porosity.
Transcript #
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[Music]
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Welcome back to a new episode of Prep, Set, Go and we'll be continuing our look at surface
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preparation and today we are going to talk about substrate porosity - so what it means,
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why it's important, and how this can impact the application of our surface finish. So let's get
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started. So first off what is surface porosity? Well porosity relates to the absorbency or
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permeability of a surface to moisture and gases. So highly porous surfaces readily absorb moisture,
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and can include surfaces like screeds, render, or plasterboard to name a few.
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While non-porous substrates don't readily absorb moisture, and can include things like metals,
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plastics, glass, and ceramics. Now concrete can fall into both categories,
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and its porosity is determined by a number of factors including the use of densifiers
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in the mix, the strength of the mix, and how the concrete is finished or floated. So with
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steel floats and power finish as they produce a smoother more burnished finish. So why is
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assessing surface porosity important when it comes to bonding a finish or a coating?
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Now for that we need to look at how the surface porosity can actually directly affect our overlaid
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coating, it can determine the suitability of a surface for applying a coating or finish over, and
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it can determine the type, and level, of surface preparation that may be needed. So let's look at
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how the porosity of a surface can directly affect our overlaid system. So number one, adhesion.
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Porous surfaces are typically easier to bond to because of the open surface profile, on the
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flip side however, for liquid coatings including waterproof membranes, they're more susceptible to
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pin holing when applied over a porous surface. Now this is due to the rapid absorption and
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drawing down of the moisture contained in the material into the actual substrate. Pores on
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concrete contain air and become trapped when the coating is applied over it, the trapped
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air then tries to escape pushing up against the coating and then creating these small pin holes.
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Three, the coverage rates. Now, porous surfaces will generally require greater coverage of a
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coating or an adhesive. Drying times - Now liquid coatings and cement-based adhesives will dry much
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faster over a porous surfaces as the moisture is drawn out at a much faster rate. Now let's look
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at non-porous surfaces. Number one, adhesion. Now non-porous surfaces are generally more challenging
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to adhere to. Coatings or adhesives if unsuited for use over these surfaces may eventually dry but
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not achieve any meaningful bonds to the surface to the point where they can actually just be
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peeled up cleanly from a surface. Two - Surface Prep. Now depending on what's being applied,
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non-porous surfaces can require a greater level of preparation and can include things like mechanical
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treatment, grinding to open up the pores and roughen the surface. Number three - Drying.
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Liquid coatings would take longer to dry over less porous surfaces, as the rate of absorbency
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is reduced. Now ultimately the suitability of a surface for the application of a coating or
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finish based on its porosity is determined by the manufacturer of the system to be applied.
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So before starting any application it's crucial that you read the manufacturer's recommendations
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and user instructions found on the product TDS or packaging. So the obvious question, how do
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you determine the porosity of a surface? Now the standard ASTM f3191 establishes a simple method
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for determining surface porosity by way of a water droplet test. And the method is as follows:
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so substrates must be clean - free of dust and other surface contaminants. Place a single drop
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of clean water onto the surface, and the porosity of the substrate is then determined by how long
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the water takes to absorb into the substrate. So according to the standards guidelines,
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less than one minute is a porous substrate, and more than one minute is a non-porous substrate.
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Now surface preparation. As mentioned earlier the relative preparation required for porous
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and non-porous surfaces is determined by the manufacturer of the system to be applied.
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Now over porous surfaces for liquid coatings and waterproof membranes, a primer would typically
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be required. Now the primer will soak into the surface and then displace the trapped air - in
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turn reducing the likelihood of pin holing. The primer may also help regulate drying by
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preventing the rapid absorption and drawing down of moisture from the coating or adhesive.
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The primer can also help improve coverage by preventing the overabsorption of moisture into
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that surface. Now for non-porous surfaces, primers can also be used to provide a compatible surface
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for overlaid coatings and finishes, and enable to bond to a surface that wouldn't normally be
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possible. Alternatively, mechanical preparation may be required to provide the necessary key,
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and then can include things like grinding or shot blasting. So thanks again for joining us on
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for this episode of Prep, Set, Go, we hope you've taken something away from it, and of course if you
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have any questions, comments, or feedback, we'd love to hear from you. Until next time, goodbye.
