Short answer: Most microcement damage can be repaired locally. Fill hairline cracks and chips with Microcement Repair Mix at a yoghurt-like consistency, let it firm up for a couple of hours, sand it flat with a fine grit, then re-seal. Cured sealer acts as a primer, so recoating only needs a light sand first.
Microcement is tough, but floors take knocks, worktops meet dropped pans and older buildings move. The good news is that you rarely need to start again. This week alone we had phone calls about a hairline crack in a floor in a 1705 cottage, a trade team redoing an area that was already sealed, and a customer wanting a different sheen over matt sealer. Here's exactly what we told them.
Quick facts
| Job | Time on the tools | Difficulty | You'll need |
|---|---|---|---|
| Hairline crack | Under an hour, plus drying | Easy | Repair Mix, spatula, fine sanding discs, sealer |
| Chip or small hole | An hour or two over a day | Easy | Repair Mix, spatula, sanding discs, sealer |
| Recoat or change finish | A day, plus a week of sealer cure | Moderate | Sanding discs, sealer (and microcement if recoating) |
Is it a repair job, or is something still moving?
Before you fix anything, work out why it happened. A one-off chip from a dropped tin is a simple repair. A crack that runs in a straight line, keeps growing or follows a board joint usually means the substrate underneath is moving. Fill that without fixing the cause and it will come back.
If you're not sure, our guide Do Microcement Floors Crack? explains the usual causes, from flexible floors to missing mesh. Send us a photo at info@concretelab.co.uk and we'll tell you which it looks like.
How do you fix a hairline crack in microcement?
- Clean it out. Brush or vacuum the crack so there's no dust or grit in it.
- Mix a small amount. Mix a little Repair Mix to a yoghurt consistency. You only need enough for the crack, so don't mix a big batch.
- Scrape it in. Use a flexible spatula held at a high angle to force the mix into the crack, then scrape the surface clean so you're not leaving a thick layer on top.
- Let it firm up. Wait a couple of hours.
- Sand lightly. Use a fine (high) grit to take it flush with the surrounding surface.
- Re-seal. Apply sealer over the repair (see below).
Don't prime first. It's impossible to get primer into a hairline crack, and you don't need to.
How do you repair a chip or a small hole?
Chips are the same process with one difference: depth. Microcement and repair mix are designed to go on thin. If you pack a deep hole in one go, it may crack as it dries.
- The odd small hole or chip: fill it with Repair Mix in thin passes, letting each one firm up before the next, then sand flat.
- Lots of holes, or anything around 5mm deep: fill and level them first with a suitable filler, then apply microcement or Repair Mix over the top.
- Edges and corners: leave the repair slightly proud, then sand it back to a crisp line once it's firm.
Can you put new microcement or sealer over old sealer?
Yes. This was a question from a trade flooring team this week. Once Concrete Lab Sealer has cured, it acts as a primer for whatever goes on next. You don't need to strip it off.
- Give the sealed surface a light sand to key it. You're scuffing the sheen, not cutting through to the microcement.
- Vacuum and wipe off all the dust.
- Go straight over the top with microcement (if you're recoating) or with sealer (if you're refreshing the finish).
Can you change from a matt to a satin finish?
Yes, that's absolutely fine. Lightly sand the matt sealer, clean off the dust and apply your satin sealer coats. If you're after the most natural concrete look, matt is the one we usually recommend, so try the new finish on a small area first.
How do you re-seal a repaired area?
Concrete Lab Sealer is a two-part sealer applied in three coats. Use the sealer calculator to get the mix and dilution right for each coat.
- Leave at least four hours between coats. There's no maximum, so you can come back the next day.
- Feather the sealer out beyond the repair so you don't get a hard edge in the sheen. On a small area it often looks better to re-seal the whole panel or board, edge to edge.
- The sealer reaches full cure after seven days. You can use the surface before then, but go gently: no heavy furniture dragged across a floor, no hot pans or harsh cleaners on a worktop.
Remember that microcement itself is waterproof. The sealer is there for stain resistance and durability, which is why a scuffed or worn sealer is worth refreshing even when the microcement underneath is fine.
Will the repair match the colour?
Repair Mix comes in our colour bases and is made to blend with Concrete Lab Microcement. Microcement is a hand-applied, natural-looking finish, so a repair is rarely completely invisible, especially in raking light. Do a test patch somewhere discreet first and sand gently to blend the edges.
If your microcement came from another brand, the same method works, but colour matching is harder. Order a colour sample and compare it in your own light before you start.
Is a crack covered by the guarantee?
Our 12-month product guarantee covers product failure, including hairline cracking, when the microcement has been applied following our instructions. Send us photos and a description of the substrate and we'll take it from there.
Want to stop it happening again? Read how to clean and maintain microcement floors and how to look after a microcement worktop.
Frequently asked questions
Can you repair microcement without redoing the whole floor or wall?
Yes. Hairline cracks, small chips and worn patches can be repaired locally with Microcement Repair Mix, sanded flat and re-sealed. You only need to strip back and recoat when the damage is widespread or the substrate is still moving.
Do I need to prime before filling a hairline crack?
No. You can't get primer into a hairline crack. Clean it out, then scrape repair mix in at a yoghurt consistency.
Can I put new microcement or sealer over old sealer?
Yes. Cured Concrete Lab Sealer acts as a primer. Give it a light sand to key it, clean off the dust and go straight over the top.
Can I change from a matt to a satin finish?
Yes. Satin sealer can go over a matt-sealed surface. Lightly sand, clean and apply your new sealer coats.
How long before I can use a repaired area?
Leave at least four hours between sealer coats. The area is usable once the sealer is dry, but Concrete Lab Sealer takes seven days to reach full cure, so treat it gently for that week.
Is a crack covered by the guarantee?
Our 12-month guarantee covers product failure, including hairline cracking, when the product has been applied following our instructions. Check the guarantee page or email us with photos.
Still not sure? We're happy to talk you through it — info@concretelab.co.uk
