Description
A quartz key coat decides whether a thin coat render grips the wall or slides on it. Cerplast is the Atlas answer: a ready-to-use acrylic mass filled with quartz flour, rolled or brushed over a cured basecoat so the finish has something rough and evenly absorbent to bite into. Renders World stocks it in four pack sizes, of which this white 25 kg tub is the largest, and it belongs with the render primer range rather than with the thin penetrating emulsions.
What a Quartz Key Coat Does Between Basecoat and Finish
Four things make the case for this key coat, and only one of them is adhesion.
- Grip. The aggregate develops the effective surface between the two layers and leaves a deliberately rough film — what the wet render keys into, and what stops it slipping as it is pulled up.
- Even suction. Cerplast limits how fast the substrate pulls water out of fresh render, so a patchy wall stops behaving like several walls at once.
- A chemical barrier. The dried film is described as a barrier between substrate and render that limits colour bleed from below and the stains it produces on the finished surface. Under a light finish on a repaired or discoloured wall, that is usually the property which earns the money.
- A named component, not an approximate match. Cerplast is listed by name in both product sheets of a UK system certificate, so a specification can point at the document rather than at a description.
Adhesion to concrete is stated twice in the same technical sheet, and not identically: the properties paragraph gives min. 1.0 MPa, the technical data table gives > 1.0 MPa. At least 1.0 MPa holds either way, and that is the figure to put in a specification.
Cerplast in Numbers — the Manufacturer's Own Figures
| Property | Documented value |
|---|---|
| Composition | Acrylic resins and quartz flour |
| Density, ready to use | Approx. 1.5 g/cm³ |
| Adhesion to concrete | At least 1.0 MPa |
| Consumption | 0.3 kg/m² on average |
| Drying before rendering | 4 to 6 hours |
| Working temperature | +5 °C to +30 °C |
| Application | Roller, brush or machine |
| Pack sizes | 5, 10, 15 and 25 kg buckets |
| Shelf life | 12 months from production date |
| Colour | White base, seven DEKO M shades, SAH tinting |
| Rate inside BBA 13/5018 | 0.25 to 0.35 kg/m², system scope |
Coverage and How Many Buckets a Wall Takes
Consumption is declared per square metre and tubs are bought whole, so the arithmetic is worth doing once.
- At the average rate of 0.3 kg/m²: 25 ÷ 0.3 = about 83 m² from one bucket.
- At the upper rate the BBA certificate applies to key coats inside its build-up, 0.35 kg/m²: 25 ÷ 0.35 = about 71 m².
Order against the higher rate and the wall never runs short: 100 m² works out at 100 ÷ 71 = 1.4, so two tubs, with the balance absorbed by reveals and rough patches. On a smooth, well-prepared basecoat the average rate is realistic and one tub covers most of a semi-detached gable.
Which Finishes the Documents Allow — and Where They Disagree
The paperwork splits here, and the split is worth understanding before the order goes in.
- The current technical sheets — Polish edition updated May 2025, English edition July 2023 — put Cerplast under thin coat renders described as mineral, acrylic, silicone and mosaic DEKO M.
- BBA Agrément Certificate 13/5018, Product Sheet 1, for the EPS system, narrows that: Cerplast is the key coat for Cermit Mineral, Atlas Acrylic Render and Acrylic-Silicone Render, while silicone and silicone-silicate finishes take a different, silicone-based key coat named in the same sheet.
- Product Sheet 2, for the Roker MW mineral wool system, narrows it further: Cerplast pairs with Cermit mineral and nothing else.
Both positions come from the same manufacturer, and the difference is scope rather than error: a product sheet describes what the material can do, a certificate what one tested assembly contains. On a job running to a certified build-up, follow the certificate — it permits less, and a warranty follows the assembly that was tested. Outside one, the technical sheet governs. The primer comparison for thin coat systems sets out how they divide by finish type.
Colour: One White Base and Seven Matched Shades
Cerplast leaves the factory white, which is why it works as a barrier under pale finishes, and Renders World tints it at the counter while an order is made up. Beyond white it is made in seven shades matched to the DEKO M mosaic renders:
- White, brown, clinker, graphite, sand, grey and coffee — the shade for a particular DEKO M pattern is printed in that render's own technical sheet.
- Tinted to the SAH Colours of Renders and Paints: 400 shades on the July 2023 English sheet, 418 on the May 2025 Polish one. Specify against 400 and the shade exists in either.
Preparing the Wall Before the Key Coat Goes On
The substrate has to be stable, air-dry, even and clean, and each of those has a named route.
- Let it season. New cement and cement-lime plasters need at least 7 days and moisture of 4 % or less; concrete, at least 28 days and structural moisture below 4 %. Both assume +20 °C and 50 % humidity, so a cold damp October adds time. A reinforced layer built with an Atlas adhesive takes its seasoning time from that adhesive's own sheet — where the 2023 English edition gave a flat three days.
- Make it even. Cavities and unevenness are filled with a levelling mortar such as ZW 330, and the repair is primed first: Atlas Uni-Grunt, the concentrated Uni-Grunt Ultra, or the ready-to-use NKP primer, which needs no dilution.
- Clean it back. Dust, dirt, lime, oil, grease, wax and residues of oil and emulsion paint all come off. Where fungi or algae have taken hold, they go first, with Mykos Plus.
Where one elevation mixes brick, block and old render, the suction differences govern the result; the substrate matrix for brick, block and painted walls covers how each behaves, and the wall preparation method covers the sequence.
Applying Cerplast — Conditions, Sequence and Drying
Cerplast arrives ready to use and must not be combined with other materials, diluted or thickened. Stir it to an even consistency immediately before use and again while working, then spread it evenly across the whole surface with a roller, a brush or a machine. Rendering or fixing cladding begins once the film has dried completely, 4 to 6 hours later.
- The temperature window covers three things at once — mass, substrate and air — and the substrate is the one that catches people out on a bright cold morning. Working round from the shaded side keeps all three inside +5 °C to +30 °C for longer.
- Shade the drying film. The primed surface is protected from direct sun, wind and precipitation during the work and while the render above it dries.
- A pause is allowed for. Where the season ends and the weather will not let the finish go on, a reinforced layer may be left protected by Cerplast alone until conditions allow the render. The current sheets attach no period to that; the 2015 English edition put it at half a year, the only figure any edition has given.
- Clean tools with clean water immediately, before the acrylic sets in the sleeve.
The quartz key coat application guide for EWI systems goes further into tool choice and weather planning.
What BBA 13/5018 Covers, and What It Does Not
Agrément Certificate 13/5018 belongs to a whole external wall insulation system, in two product sheets: the EPS one, fourth issue October 2022, and the Roker MW one, second issue December 2018. Cerplast appears inside both as one named component of an assembly, which is a narrower and more defensible claim than anything said about a tub on its own.
- Impact resistance, mineral wool. Roker W-20 basecoat with Cerplast and a Cermit mineral finish is listed at 7 mm and rated Use Category III — a figure for the build-up, not for the primer.
- Impact resistance, EPS. No equivalent figure exists, and the reason is the shape of the table rather than the product: its rows name the basecoat and the finish but not which primer went with them.
- Durability. Installed and maintained to the certificate holder's recommendations, the system is assessed to remain effective for at least 30 years.
No reaction-to-fire class is quoted for the primer on its own. Those classifications belong to complete build-ups of named adhesive, board, basecoat, mesh, primer and finish, and lifting one onto a single component would misdescribe what was tested.
The 25 kg Bucket Against the 15 kg
One formulation in two pack sizes, so the choice is purely a quantity decision.
| Pack | Coverage at 0.30 to 0.35 kg/m² | When to order it |
|---|---|---|
| 25 kg tub | About 83 to 71 m² | Full elevations, whole-house runs |
| The 15 kg tub | About 50 to 43 m² | A single elevation, reveals, top-ups |
For a mid-size semi, one large tub plus one small is tidier than three small ones: every tub tinted at the counter is its own batch, and fewer batches means fewer chances of a visible edge. Renders World processes orders the same day where the stock is there, and the delivery cost is worked out by the team once the order is in rather than at checkout. The other quartz primer on this shelf, Ceresit CT 16, belongs to a different manufacturer's system and carries its own colour-matching instruction.
Quantity, Compatibility, Conditions and Storage
How many tubs does a 100 m² elevation take?
Two on the certified rate of 0.35 kg/m²: 100 ÷ 71 = 1.4, rounded up. At the average 0.3 kg/m² the same tub reaches about 83 m².
Can Cerplast go under a silicone render?
The current technical sheets include silicone among the renders it primes; both product sheets of the certificate send silicone and silicone-silicate finishes to a silicone-based key coat instead. Inside a certified assembly, follow the certificate.
What conditions does application need?
Three numbers govern the day:
- Mass, substrate and air all between +5 °C and +30 °C.
- No direct sun, wind or rain on the primed surface while it dries.
- Four to six hours before the render goes on — an overnight gap is a sound habit on a damp autumn job.
How long does an unopened tub keep?
Twelve months from the production date printed on the packaging. Storage and transport conditions are sent to the product's safety data sheet rather than the technical one; the 2015 English edition stated that the mass freezes and irreversibly loses its performance below 0 °C, so a frost-free store is the sensible reading over a British winter.
Technical Documentation for Atlas Cerplast
- Cerplast technical data sheet, Polish edition updated 2025-05-22 (PDF) — the current edition.
- Cerplast technical data sheet, English edition updated 2023-07-24 (PDF)
- BBA Agrément Certificate 13/5018, Product Sheet 1 — Atlas/Aval EPS system (PDF)
- BBA Agrément Certificate 13/5018, Product Sheet 2 — Atlas/Aval Roker MW system (PDF)