Microcement Hairline Cracks After Curing
moderate 2–5 h
Tools: raking light (a torch held low), fine sanding pad (220–400), stainless trowel, vacuum, lint-free cloths · Materials: microcement fine finish coat matching the colour, the system's primer, two-part polyurethane sealer
Steps
- Look at the cracks in raking light and press them: crazing that does not catch a fingernail is in the sealer and finish coat and can be re-skimmed; a crack with a step you can feel has come through from the substrate.
- Check the sealer: a thin or patchy sealer coat is what usually lets crazing show — count coats against the system's spec (typically two, sometimes three in a shower).
- Lightly sand the affected area with a fine pad, vacuum, and wipe with a barely damp cloth.
- Apply the system primer, then a thin coat of the fine finish microcement with a stainless trowel, working the cracks flat; let it dry slowly out of sun and draughts.
- Sand smooth once cured, then apply the sealer in the number of coats the maker specifies, allowing the full time between coats.
- Keep water and heavy use off the surface for the sealer's cure — a week for most two-part polyurethanes.
Stop and call a pro if
- cracks run straight and follow tile or board joints beneath — the substrate is moving and no skim coat will hold
- the surface is a shower or wet area and water is already getting behind it
- crazing covers most of the surface within weeks of installation — that is an application fault under warranty
What a pro does that you can't
A microcement applicator identifies the layer that failed, strips back to a sound base, and reapplies with a mesh-reinforced base coat and controlled curing; on warranted work this is the installer's job.
DIY cost: $60–150 in finish coat and sealer