Fibre Reinforced Facade Encyclopedia: Finishes
An acid etched finish is produced by washing the cast face with a dilute acid that dissolves the calcium hydroxide and calcium carbonate in the surface cement paste, then neutralising and rinsing. What is left is the fine sand of the matrix and the tips of any finer aggregate, in a shallow, even, sandy texture. It is a chemical process rather than an abrasive one, which is why it holds crisp arrises and fine mould detail where blasting would soften them.
Acid etching and sandblasting both remove surface cement paste, but by different means and to different depths. Etching dissolves roughly 0.1 to 0.5 mm and keeps sharp edges; blasting cuts 0.3 mm to several millimetres and rounds them.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Mechanism | Acid dissolves calcium compounds in the paste | Chemical, not abrasive |
| Acids used | Dilute hydrochloric, phosphoric or buffered gel | Concentration and contact time set the depth |
| Light etch | 0.1 to 0.2 mm | Removes laitance only; very fine sandy read |
| Medium etch | 0.2 to 0.4 mm | The standard architectural etch |
| Practical depth ceiling | About 0.5 mm | Beyond this the acid undercuts sand grains |
| Gloss achieved | Under 5 gloss units at 60 degrees | Fully matte |
| Neutralising | Alkaline wash then clean water rinse | Residual acid causes later staining |
| Cost premium | Typically 10 to 25 percent over off-form | Effluent treatment is the variable |
| Programme | Typically 0.5 to 1 day per panel run | Done after cure, not green |
How the finish is achieved
Cement paste at a cast face is rich in calcium hydroxide and, after exposure to air, calcium carbonate. Both react readily with dilute acid and go into solution, while silica sand, quartz and most decorative aggregates do not. Etching therefore removes matrix selectively and leaves the harder grains standing, which is why the result reads as a fine even granular surface rather than a scoured one.
The acid is applied by brush, roller, low-pressure spray or as a thickened gel, left for a contact time measured in minutes, and then stopped. Stopping it properly is the whole job: the surface is flooded with water, washed with an alkaline neutraliser, and rinsed until the run-off tests neutral. Acid left in the pores keeps working, draws salts to the surface and produces efflorescence and patchy discolouration, so a panel that is etched and not properly neutralised looks acceptable on the day and poor in three months.
Depth is controlled by concentration and contact time, but only up to a point. Once the paste between the sand grains has gone, further acid undercuts and loosens those grains rather than cutting deeper, so the surface sheds sand instead of etching. That self-limiting behaviour makes acid etching a shallow finish and also makes it more repeatable across a large area than blasting.
Which materials it suits
Acid etching is the standard fine finish for architectural precast and for glass fibre reinforced concrete. On a sprayed GRC skin it is arguably the safest of the removal finishes, because the depth is inherently shallow and self-limiting so there is little risk of reaching the fibre-rich layer. It also sits most comfortably with a moulded, three-dimensional panel, since it needs no line of sight from a nozzle and reaches into reveals, returns and relief that a blast head cannot treat evenly.
The aggregate has to be acid resistant. Quartz, granite and most river gravels are. Limestone, marble and dolomite are calcium carbonate, so the acid attacks the stone as readily as the matrix and the finish comes out pitted; calcareous mixes should be blasted instead. Etching is not used on fibre reinforced polymer, where the face is a gel coat, and should not be used on glass fibre reinforced gypsum, which is calcium sulphate and water sensitive.
Acid etched compared with sandblasted
- Depth: etching is limited to roughly 0.1 to 0.5 mm; blasting runs from 0.3 mm to several millimetres and can be taken through to an exposed aggregate finish
- Edges and detail: etching keeps arrises crisp and preserves fine mould relief; blasting rounds arrises and softens detail
- Texture: etching gives an even fine sandy face, blasting an open granular slightly pitted one
- Consistency: etching is self-limiting so it is more repeatable across large areas and between operators
- Geometry: etching reaches into reveals and relief that a blast nozzle cannot treat evenly
- Waste: etching produces acidic effluent to neutralise, blasting spent media and dust to contain
Appearance, service and specifying
Etching lightens and flattens the colour, because it removes the dense surface skin and replaces it with a rough face that scatters light. An integrally pigmented mix reads a shade paler than off-form, so the oxide pigment dose should be set against an etched sample. Etching also removes the laitance in which most surface colour irregularity sits, which is one reason it is chosen: it improves apparent colour consistency across a run.
Cost is comparable with blasting and the programme impact is small, since etching can be done after cure rather than in a fixed window. In service the shallow texture breaks up run-off enough to reduce hard streaking without holding as much dirt as a deep exposure, and the surface accepts hydrophobic impregnation readily. It is not resistant to acidic cleaning products, so maintenance must specify pH neutral cleaning.
Nominate the finish as a light, medium or heavy etch with the nominal depth in millimetres, require the acid type to be declared, require full neutralising with a documented rinse-to-neutral check, and require the sand and aggregate to be acid resistant. Then sample: a control sample of at least 600 by 600 mm from the production mix by the production method, presented as a range of acceptable extremes, and a full-size first-off panel viewed vertically outdoors.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (finishes, sampling and acceptance)
- PCI MNL-117 Manual for Quality Control for Plant Produced Architectural Precast Concrete
- PCI Architectural Precast Concrete, colour and texture selection guide (etch depth categories)
- AS 3610.1 Formwork for concrete, surface finish classes
- AS 2758.1 Aggregates and rock for engineering purposes (aggregate acid resistance)
- Relevant state trade waste requirements for neutralising and discharging acidic rinse water
Related terms
Questions specifiers ask
How deep does acid etching go?
Between about 0.1 and 0.5 mm. The process is self-limiting: once the paste between the sand grains has dissolved, more acid undercuts the grains rather than cutting deeper, so the surface sheds sand instead of etching. That shallow ceiling makes it safe on a thin GRC skin.
What is the difference between acid etching and sandblasting?
Mechanism and depth. Etching dissolves cement paste chemically to roughly 0.1 to 0.5 mm and keeps arrises and fine relief crisp. Blasting abrades paste away mechanically from 0.3 mm to several millimetres and rounds edges. Etching is more repeatable over large areas; blasting goes deeper.
Why does neutralising matter so much?
Acid left in the pores keeps reacting, draws soluble salts to the face and produces efflorescence and patchy discolouration weeks or months later. The panel must be flooded, washed with an alkaline neutraliser and rinsed until the run-off tests neutral, and that check should be documented.
Can any concrete be acid etched?
Only if the sand and aggregate are acid resistant. Quartz, granite and most river gravels are fine. Limestone, marble and dolomite are dissolved by the acid along with the matrix, giving a pitted result, so calcareous mixes should be blasted instead.
Does acid etching change the colour?
It lightens and flattens it, because the dense surface skin is removed and the rough face scatters light. It also removes the laitance where much surface irregularity sits, so it usually improves apparent colour consistency. Set the pigment dose against an etched sample.
Etched, blasted or exposed aggregate? We can make the same mix up three ways so the decision is made on real panels rather than a swatch. Ask for comparison samples
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.