How to prep a surface for microcement

Prep for microcement is six steps: wipe the surface clean, make sure it is dry and sound, sand or grind it, vacuum it, prime it, and broadcast kiln-dried sand into the primer while it is still wet.

That is the whole list. None of it is difficult. It is also where most DIY microcement jobs go wrong, because every one of those steps is boring and invisible in the finished surface — right up until the finish lifts off in a sheet.

Here is what each step is actually for, and what to check before you open anything.

Clean first, because everything else depends on it

Wipe the surface clean. Whatever is on it — dust, grease, old cleaning product residue, the film that builds up on a bathroom wall over ten years — is a release agent between your substrate and your primer.

This matters most in the rooms people most want to microcement. Kitchen splashbacks carry cooking film. Bathroom walls carry soap residue. Both look clean and neither is. Floors near an entrance carry whatever has been tracked in and ground down.

Do not skip ahead to sanding on a dirty surface either. Sanding a greasy wall drives the grease into the scratch pattern you just made, which is worse than where you started.

Dry and sound — what "sound" actually means

Dry is the easy half. The surface has to be dry before primer, not damp, not recently washed and wiped.

Sound is the half people wave through, so let us be specific. A sound surface is one that is not going to move, and is firmly attached to whatever is behind it. In practice:

• Nothing flakes. Drag a scraper or a fingernail across it. If paint, render or old coating comes away, it is not sound — and microcement bonded to a flaking layer will come off with that layer.
• Nothing sounds hollow. Tap across tile and render with a knuckle or the handle of a screwdriver. A dull, hollow note means the layer has already debonded from the substrate. It is holding on by friction and habit.
• Nothing rocks or flexes. Press on it. Loose tiles, springy floorboards, a panel that gives under hand pressure — all of it moves, and microcement does not bend.
• Nothing is actively cracking. A crack that is still opening is a crack that will telegraph through your finish, no matter how many coats sit on top of it.

If the surface fails any of those, fix that first. Take off the loose material, re-fix the loose tile, deal with the movement. Prepping over a problem is the single most expensive mistake in this whole process, because you find out after all seven coats are down.

Sand or grind, then vacuum

Sand or grind the surface. You are doing two things: knocking off anything proud — high spots, lumps, ridges, old adhesive — and opening up a slick surface so the primer has something to key into. Glazed tile, sealed concrete and gloss paint are all far too smooth to bond to as they are.

Then vacuum. Not sweep, not wipe with a dry cloth that just redistributes it — vacuum. The dust you just made is a fine powder sitting between your substrate and your primer, and it does the same job as the grease you cleaned off in step one.

You will do this pairing more than once. Sand and vacuum before the build coat. Sand or grind and vacuum before the microcement. Light sand, vacuum, then a microfiber wipe before the sealer. The sequence repeats because the principle does: nothing bonds to dust.

Prime, then broadcast sand while it is wet

Primer is single part and ready to use — shake it well, no mixing, no thinning. Roll one even coat. Keep it uniform. No puddles, no heavy spots, no dry patches you meant to come back to.

Then, while that coat is still wet, broadcast kiln-dried sand over it. This is the step that gets skipped and it is the one that matters most. The sand embeds in the wet primer and sets into a gritty, mechanical key. What goes on next is not relying on chemical adhesion alone — it has something physical to lock into.

Primer covers 600–650 sq ft per pail and cures in 30 minutes to 4 hours before the next step. It is the fastest layer in the system, and it is one coat only.

What you are going over decides which system you need

Prep is not one-size-fits-all, because the substrate changes what has to go on top of it. This is the part to settle before you buy anything.

What you are going over	System
Interior walls	Core
Bare concrete floors	Core
Indoor fireplaces	Core
Anything wet — showers, wet rooms	Reinforced
Anything outdoors	Reinforced
Any floor that is not bare concrete	Reinforced
Old tile	Reinforced

Core is primer, microcement, sealer. Reinforced adds fiberglass mesh and a two-coat build coat between the primer and the microcement, because those surfaces either move, get wet, or have joints underneath that would otherwise show through.

Old tile is the clearest case. Microcement goes straight over it — you do not need to rip it out — but the grout lines are channels in the surface and tile moves differently from what it is stuck to. That is a Reinforced job, every time. Our guide to what microcement can go over covers the substrates individually, and the systems page is where to confirm which kit your job needs.

The extra prep step in a Reinforced job

If you are on Reinforced, there is prep between layers too. Once the primer has cured, sand it and vacuum before the mesh goes on.

The mesh is fastened with staples or a glue gun and then pulled tight — all slack out, all wrinkles out. A wrinkle in the mesh is a ridge in the build coat, and a ridge in the build coat is a ridge in your finish. One roll covers 500 sq ft.

Then the build coat gets troweled into the mesh and pulled tight, with sand broadcast while wet, same principle as the primer. Two coats, and a 12-hour cure after the second.

Time this properly before you start

Prep is unglamorous and it takes longer than people budget for. Cleaning, grinding and vacuuming a floor is most of a day on its own for a decent-sized room, before a single pail is opened.

Then the coats stack up. A full Reinforced job is seven coats with two 12-hour cures in it, which makes it realistically a 2–3 day job. Core is shorter but still not an afternoon. Work out your quantities first with the material calculator so you are not stopping mid-job to order, and if you are still pricing the project, what microcement costs per square foot has the full numbers.

One last thing worth saying plainly: you can buy all of this without a certification, a trade account, or any proof you work in the trade. Almost all microcement in the US is sold only to certified applicators. Which means the prep advice is usually held back too — so read it twice, and do not cut the steps nobody will see.

Microvella ships late 2026. Join the waitlist and we will email you the day it is ready to order.