Description
This is the larger of the two Uni-Grunt containers stocked in the Renders World exterior render primers range, and every figure below is taken from the Atlas Uni-Grunt product data sheet and the live product endpoint, both read today. The 100 m² in the address is not one of them.
Where a Deep-Penetrating Primer Earns Its Place on Absorbent Walls
The Atlas Uni-Grunt 10kg wall primer is a water-dispersed polymer emulsion consuming 0.05–0.2 kg/m² depending on substrate absorbency, so one container primes between 50 and 200 m² of masonry before adhesive, render or paint goes on. That spread is the point of the product rather than a vagueness in it: the sheet's stated purpose is to decrease and unify substrate absorptiveness, so a thirsty patch drinks more emulsion and then behaves like the sound brickwork around it.
The data sheet lists what that buys on the next layer, and three of the seven properties matter on a rendering job:
- Unified setting conditions — the sheet's wording is that the product applied as the next layer has uniform setting conditions over the whole surface, including areas with local repairs. On a patched elevation that is the difference between an even finish and a map of the old repairs.
- Reinforced surface — the emulsion penetrates the structure of mineral substrates, reinforces it and improves its load-bearing capacity, and binds non-stable substrates by reducing dust on plasters and screeds.
- Improved yield above — by sealing the substrate structure it increases the yield of the material applied next and reduces its consumption, which shows up in the render order rather than the primer order.
The sheet also names adhesives for thermal insulation among the layers it prepares the wall for, so on an external wall insulation job this is a masonry primer used before the board adhesive. It is not the key coat over a reinforced basecoat: in BBA Agrément Certificate 13/5018 the key coats named for the Atlas/Aval EPS and Roker MW systems are Atlas Cerplast, Atlas Silkat ASX and Atlas Silkon ANX, and that is a separate layer with a separate job.
Why Applicators Specify the 10 kg Container
- Fifteen minutes to an Atlas adhesive mortar: the sheet allows the next layer after approximately 15 minutes for Atlas adhesive mortars, so priming and board fixing sit in the same morning.
- Roughly two hours to general finishes: painting, wallpapering and tiling can commence once the emulsion dries, approximately two hours after application, with the sheet naming self-levelling floors and painting coats on cement-lime plasters, silicate hollow blocks and aerated concrete blocks in that group.
- 1.92 g/l VOC against a 30 g/l limit: declared Cat. A/h/FW on the sheet, which describes it as a fifteen-fold reduction against the legal requirement — useful on occupied and poorly ventilated interiors.
- Colourless and solvent-free: transparent and water-soluble in the wet state, colourless after drying, and the sheet states it does not cause substrate discolouration, which matters under a pale render or a mineral paint.
- Dilutable rather than fixed: the same emulsion is used undiluted, at 1:1 for a first coat on very absorptive substrates and at 1:3 on new gypsum finish coats indoors.
- Cheaper per square metre than the small pack: £20.90 for 10 kg against £12.50 for 5 kg, both fetched August 2026, works out at £0.10–£0.42 per m² against £0.13–£0.50 per m² on the same consumption band.
Technical Data Read From the Atlas Uni-Grunt Sheet
| Property | Value as documented |
|---|---|
| Composition | Impregnating sealer based on a water dispersion of polymer resins |
| Density of emulsion | Approx. 1.0 g/cm³ |
| Average consumption | 0.05–0.2 kg/m², depending on substrate absorptiveness |
| Substrate and ambient temperature during application | +5 °C to +30 °C |
| Further work — Atlas adhesive mortars | After approx. 15 minutes |
| Further work — self-levelling floors, painting coats on cement-lime plasters, silicate hollow blocks, aerated concrete blocks | After approx. 2 hours |
| Appearance after drying | Colourless, matt |
| VOC | 1.92 g/l maximum content; permissible content 30 g/l (Cat. A/h/FW) |
| Application | Roller, brush or sprayer; indoors and outdoors, horizontal and vertical |
| Packaging | Plastic containers of 1 kg, 4 kg, 5 kg and 10 kg |
| Shelf life | 12 months from the production date shown on the packaging |
| Classification | Not classified as a construction material |
| Other certification | Radiation Hygiene Certificate, named on the sheet without a number |
That last classification line is worth reading twice before an inspector asks. Because the primer is not classified as a construction material, there is no declared performance and no DoP number to quote for it — the data sheet is the document, and the certified performance on an insulated wall belongs to the system above it.
How to Apply Uni-Grunt: Substrate, Dilution and Coats
Preparation and the dilution the sheet sets
The substrate wants to be dry and free from dust, dirt, oils, grease and wax, with any loose or poorly bonded layers removed first. The emulsion is supplied ready to use and must not be mixed with other materials or thickened, so the only decision left on the wall is dilution:
- Standard mineral substrates: apply undiluted, once, with a roller or brush in a thin and uniform coat.
- Very absorptive and weak substrates: double priming — the first coat diluted 1:1 with water, then the priming repeated with undiluted emulsion.
- Newly made gypsum finish coats under indoor paints: apply once, diluted 1:3 with water.
One wrinkle in the sheet is worth carrying to site: the double-priming row says to repeat the priming once the first coat dries, and then labels the same operation the "wet on wet" method. The two phrasings do not agree, and the safer reading is the sequence — let the diluted coat dry, then apply the undiluted one. The general method sequence, including where a key coat follows, is set out in the substrate preparation guide.
How far a 10 kg container goes
Coverage follows directly from the consumption band, and the arithmetic is worth showing because the address on this page carries a figure the sheet does not: 10 kg ÷ 0.2 kg/m² = 50 m² on a thirsty substrate, and 10 kg ÷ 0.05 kg/m² = 200 m² on a tight one. Order against the 0.2 kg/m² end and the wall is covered either way. At £20.90 per 10 kg container, August 2026, that is £20.90 ÷ 50 m² = £0.42/m² at worst and £20.90 ÷ 200 m² = £0.10/m² at best. Where the substrate is unknown — old brickwork beside new blockwork, say — the matrix in the guide to rendering on brick, block and painted walls is the quickest way to place it before ordering.
Site Notes the Data Sheet Adds
- Some substrates are routed elsewhere by Atlas. The difficult-substrate table sends existing ceramic and stone cladding, terrazzo, OSB boards, stable linoleum and PVC, concrete varnishes and oil-colour paint coatings to Atlas Grunto-Plast or Atlas Ultragrunt instead, because those need a rough keyed film rather than penetration.
- Clean tools with clean water directly after use, before the polymer resin cures into the pile; the sheet names a dedicated Atlas remover for dried emulsion stains.
- Follow the paint maker's guidance on gypsum. The sheet defers to the paint manufacturer's own priming instruction when gypsum substrates are primed before painting, which is the one case where its dilution table is not the final word.
- Priming is preparation, not a key. A thin-coat render still wants a quartz key coat above the consolidated substrate — the two products and their very different consumption bands are compared in the primer comparison for thin-coat render, and the method sits in the quartz primer application guide.
10 kg or 5 kg: Sizing the Container to the Wall
Same emulsion, same sheet, same consumption band — the choice is arithmetic and shelf life.
| Your Project | Best Pack | Standout Spec |
|---|---|---|
| Full or multiple elevations, EWI substrate priming | Uni-Grunt 10 kg | 50–200 m²; £0.10–£0.42/m² |
| Single wall, interior room, van-stock top-up | Uni-Grunt 5 kg container | 25–100 m²; £0.13–£0.50/m² |
Two 5 kg containers cost £25.00 against £20.90 for the 10 kg, a difference of £4.10 for the same 10 kg of emulsion, both figures fetched in August 2026. Against that sits the 12-month shelf life: past about 100 m² of thirsty wall the large container is the cheaper and simpler order, and below it the small one is less likely to be half full next spring.
Is This the Right Primer for Your Substrate?
- Yes, for absorbent mineral walls — brick, silicate and cellular concrete blockwork, cement and cement-lime plasters, and patched elevations where absorbency needs equalising before render, adhesive or paint.
- Yes, ahead of an Atlas adhesive mortar — the 15-minute figure keeps board fixing on the same visit as priming.
- Add a key coat above it — a thin-coat finish bonds to a quartz coat, so pair this with one from the same Renders World primer range and check which key coat the render's own system names.
- Choose a different primer for cladding, terrazzo, OSB, linoleum, varnished or oil-painted surfaces, which Atlas routes to Grunto-Plast or Ultragrunt.
- Then choose the finish — the silicone render range from Renders World covers the topcoats this preparation is usually leading to.
Listed as available at the time of this fetch, August 2026, at £20.90 per 10 kg container as displayed on the product endpoint.
FAQ — Coverage, Dilution and Working Limits
How much wall does one container prime?
Between 50 and 200 m², because the sheet gives average consumption as 0.05–0.2 kg/m² and leaves the exact figure to substrate absorptiveness. Plan on the 0.2 kg/m² end for old brickwork and fresh sand-and-cement patching, and treat anything better as a bonus rather than a budget.
Which dilution does my wall need?
Sound mineral substrates take the emulsion undiluted in one thin, even coat. Very absorptive and weak substrates take double priming, 1:1 with water first and undiluted second. New gypsum finish coats under indoor paints take a single 1:3 coat.
Can render go straight on top of it?
The sheet's two-hour drying group names painting coats, self-levelling floors and general finishes rather than thin-coat renders, and BBA 13/5018 names Cerplast, Silkat ASX and Silkon ANX as the key coats for the certified Atlas systems. Treat Uni-Grunt as the substrate layer and follow the render manufacturer's instruction for the coat directly beneath the finish.
What temperatures does the sheet allow?
Substrate and ambient temperature both between +5 °C and +30 °C, indoors or outdoors, with drying times stated as dependent on temperature and air humidity. UK practice is to prime early on a settled day so the emulsion penetrates before the surface warms up.
Is it suitable for occupied interiors?
Maximum VOC content is declared as 1.92 g/l against a permissible 30 g/l under Cat. A/h/FW, which the sheet describes as fifteen times below the legal requirement, and the product is solvent-free, water-soluble and colourless once dry. Normal ventilation during the drying period is the sensible precaution.
Technical Documentation for Atlas Uni-Grunt