Resurfacing a concrete floor without properly repairing cracks is a recipe for disaster. Cracks will "telegraph" through fresh overlays, microcements, or coatings — ruining the appearance and causing delamination or movement failures. In this guide, we'll walk you through exactly how to assess, chase, fill, and prepare cracks before resurfacing, ensuring a solid, stable, and long-lasting finish.
Not all cracks are the same. Walk the slab and identify each crack's type: shrinkage (hairline and stable), settlement (wide and vertical movement), or structural (offset slabs, new widening). Hairline shrinkage cracks are usually safe to fill. Structural cracks need stabilisation first. Settlement cracks should be evaluated for slab movement before resurfacing. Knowing the type of crack determines how aggressive your repair method must be.
You must open cracks up before repairing them. Use an angle grinder fitted with a crack chasing blade to create a clean, 6–10mm wide U-shaped channel along the crack. This gives repair materials space to grip and lock into the slab, not just sit on top. Dust must be fully extracted during this process. Simply applying filler to the surface of a hairline crack without chasing will almost always fail under a resurfaced layer.
After chasing, thoroughly clean the crack and surrounding area. Use an industrial vacuum to extract all dust, debris, and loose material. If contamination like oil or grease is present, degrease before vacuuming. Surface cleanliness is critical — no matter how good the filler is, it cannot bond properly inside a dirty or dusty crack. Don't rush this stage; good surface prep ensures long-term stability.
For resurfacing projects, use semi-rigid epoxy crack fillers or fine-grain repair mortars. Polyurea fillers are also popular for fast turnaround times. Avoid soft, flexible caulks — they move too much under hard overlays. The filler should cure hard enough to support resurfacing without cracking again, but still flex slightly with minor slab movement. Always match filler choice to overlay type (microcement, polymer screed, etc.) for best compatibility.
Deep cracks may require filling in two stages. Force repair material deep into the chased channel using a trowel or injection method. Overfill slightly, then allow material to cure or settle. In some cases, a second application is needed to bring the surface fully flush with the surrounding concrete. Avoid leaving dips or overfilled mounds — overlays are thin and telegraph surface imperfections easily. Smooth, even fills are essential before resurfacing.
Once crack repairs have cured fully (per product instructions), lightly grind the filled areas flush with the surrounding slab. Use a hand grinder or walk-behind machine fitted with medium grit diamonds. Grinding ensures the filler doesn’t sit proud of the surface or leave raised bumps that will show through thin resurfacers. Blending repairs into the slab also helps new overlays adhere without risk of "ghosting" lines later.
After crack repairs are complete and flush, proceed with surface priming, profiling, or additional surface prep (as specified by your overlay system). Many microcements and overlays require epoxy primers or scratch coats to guarantee bond strength. Cracks properly repaired and prepped will now be invisible once resurfacing layers are applied — giving a seamless, stable floor without telegraphing or movement issues.
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