Q: "I'm planning to apply microcement over a brand-new, smooth screed. Do I need a primer, and do I really have to follow all the prep instructions?"
Great question! If you're dreaming of that sleek, polished concrete floor, especially after a new extension, it’s tempting to just dive in with the microcement. Screed’s down – let’s go, right? Not so fast! There are a few important things to consider before you grab that trowel.
Things to Think About:
Image courtesy of ejsfloorsolutions.com
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Is Your Screed Anhydrite? If you’re working with an anhydrite screed (which contains gypsum), you'll need to sand off the top layer. Why? Because this type of screed works on the basis that its very liquid and as a consequence it has a very smooth laitance which is the surface that doesn’t bond well with microcement, and without sanding, you could end up with a disaster.
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Is Your Screed Dry Enough? Patience is key here. Screed needs time to dry—about 1mm per day under normal conditions. If it’s still damp when you apply the microcement, you risk cracks, or worse, it could delaminate. Yikes!
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Got Underfloor Heating? If you have underfloor heating, it needs to be fully commissioned first. This process allows moisture to evaporate slowly from the screed. Here's a quick guide to doing it right:
- Start raising the temperature by 5°C per day.
- Max out at 26°C and keep it there for about two weeks.
- Slowly bring it back down. The whole process should take around 3 weeks.
- Make sure the substrate humidity is below 4%.
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Is the Floor Really Smooth? This is a big one. We’ve heard people say their screed is "perfectly smooth," only to discover it’s full of little divots and rough patches. If the surface isn’t truly smooth, you’ll end up using way more microcement than you planned—and potentially run out halfway through the job. Not ideal!
The Answer: Always Follow These Steps
- Primer: Yes, ALWAYS use a primer, no matter how smooth or new your screed is.
- For Anhydrite Screed: Sand off the top layer (also called laitance). This weak layer needs to go if you want a strong bond with the microcement. Plus, gypsum in the anhydrite can react badly with the cement in microcement, causing all sorts of nasty issues.
- Smooth Surface: If your screed isn’t perfectly smooth, apply a self-leveling or latex flooring compound. Remember, for anhydrite screeds, you'll need to sand off the laitance and prime before applying the leveling compound. And don’t forget to add a fiberglass mesh to prevent cracking.
- Underfloor Heating: Make sure it’s commissioned properly, so you don’t get cracks later that ruin your beautiful microcement floor.
Final Thoughts
So there you have it – the key steps to ensure your microcement project is a success. Got more questions? Don’t hesitate to reach out, we’re always here to help.
Bye for now, and happy flooring!