Preparing an existing slab
Shot-Blasting, Grinding and Scabbling Before a Screed
A bonded screed is only as good as the surface it grips. Here is how the three mechanical preparation methods differ and when each is used.
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When a new screed is bonded to an existing slab — a garage floor, a commercial unit, a thin retrofit over a sound base — the bond is the whole point. A surface skin of laitance, old adhesive, paint or dust sits between the two and stops them acting as one. Contamination like that needs to be removed mechanically before a primer goes on, following the specification of the chosen system.
There are three methods people ask about: shot-blasting, grinding and scabbling (sometimes called scarifying). They are not interchangeable. Each takes off a different depth and leaves a different profile, and choosing the wrong one either leaves contamination behind or removes more of the slab than the job needs.
This guide sets out how we think about the choice. It is written for people preparing an existing floor, not for new slabs, which have their own curing and laitance rules.
The three methods side by side
| Method | What it is suited to |
|---|---|
| Shot-blasting | Removing surface laitance and light contamination, leaving an open, even profile for primer |
| Grinding | Taking down high spots and removing coatings, adhesive residue or paint in controlled passes |
| Scabbling / scarifying | Heavier removal where the top of the slab is weak, contaminated or needs a rougher key |
Why a primer alone is not enough
It is tempting to sweep a slab, roll on a bonding primer and pour. The trouble is that primer bonds to whatever is on top — and if that is a weak laitance skin, the new screed is only as well attached as the skin is to the concrete below. That is how hollow-sounding, debonded areas appear months later.
Mechanical preparation exposes sound concrete so the primer can do its job. That is why a well-written specification states whether shot-blasting, scarifying or scabbling is included in the screeding contractor's scope or is a separate trade before them, rather than assuming someone will do it.
How we approach an existing slab
- 1
Look and tap
On survey we check for coatings, adhesive, oil, cracks and hollow areas. A tap test and a scrape tell us a lot about how sound the surface is.
- 2
Choose the lightest method that works
Laitance and dust usually need shot-blasting. Paint, adhesive and high spots usually need grinding. Weak or badly contaminated tops need heavier removal.
- 3
Vacuum thoroughly
Dust left on the surface is its own bond-breaker. Prep is always followed by a thorough vacuum.
- 4
Prime to the system
The primer is the one named for the screed going down, applied as its datasheet says.
- 5
Edge isolation and pour
Perimeter strip goes in, datums are set and the screed is pumped and levelled in a single visit.
Where this matters most on our jobs
- Garage floors, where a thin bonded screed goes over sound concrete ready for a coating
- Commercial units, where old coatings and heavy contamination are common
- Renovations where tiles have been lifted and adhesive residue remains
- Level corrections, where shallow dips take a latex skim but wider unevenness needs grinding back and a new pour
- Any bonded build-up where the finished floor height leaves no room for an unbonded screed
When bonding is the wrong answer
If a slab is heavily contaminated, cracked or has a damp problem, preparing it to take a bonded screed may not be the right route at all. An unbonded screed on a membrane separates the new floor from the old one and avoids depending on a questionable surface. We would rather talk that through on survey than prepare a slab that cannot be trusted.
Frequently asked
Existing slab? Let's see what it needs
Send a few photos of the floor and tell us what was on it. We'll say whether it needs shot-blasting, grinding or a different build-up.