Fibre Reinforced Facade Encyclopedia: Finishes
A sandblasted finish is produced by driving an abrasive medium at the cast face under compressed air so that the cement paste, and at deeper settings the fine aggregate, is cut away. The result is a matte, open, uniformly grained surface with no sheen. Depth is controlled by the operator through media, pressure, standoff and dwell, which makes it the most adjustable of the cementitious finishes and the one most dependent on operator discipline.
Sandblasting and acid etching both strip cement paste, but by different means and to different depths. A light acid etch removes roughly 0.1 to 0.3 mm chemically; a light blast removes 0.3 to 0.5 mm mechanically and leaves a coarser grain.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Mechanism | Abrasive impact under compressed air | Cuts paste and fine sand; deeper settings cut aggregate |
| Media | Garnet, copper slag, steel grit, crushed glass | Silica sand is prohibited or restricted |
| Nozzle pressure | Typically 200 to 700 kPa | Low for light texture, high for deep exposure |
| Light blast | 0.3 to 0.5 mm removed | Matte, sandy, cement colour lightened |
| Medium blast | 0.5 to 1.5 mm | Fine aggregate clearly visible |
| Heavy blast | 1.5 to 3 mm or more | Becomes an exposed aggregate finish |
| Gloss achieved | Under 5 gloss units at 60 degrees | Fully matte by definition |
| Cost premium | Typically 10 to 25 percent over off-form | Cheapest of the removal finishes |
| Programme | Typically 0.5 to 1 day per panel run | No fixed timing window after casting |
How the finish is achieved
Angular particles accelerated in an air stream strike the surface and fracture the weakest material they meet, which in a cementitious panel is the cement paste at the face. Paste goes first, then the fine silica sand of the matrix, then, at high energy, the aggregate. Each pass removes a little more, so depth is cumulative and there is no natural stopping point. That is both the flexibility of the process and its risk.
Four variables set the result. The medium sets the grain, from an even velvety texture with fine garnet to a pitted, rougher read with coarse slag or steel grit. Pressure sets the energy, and standoff, usually 200 to 400 mm, spreads or concentrates it. Dwell is the variable that ruins panels: a moment’s hesitation at a corner leaves a deeper patch that cannot be blended out, so consistent work uses a fixed standoff and a continuous sweeping motion, and high value work a mechanised head rather than a hand nozzle.
Silica sand is no longer the medium in professional practice despite the name. Respirable crystalline silica exposure is heavily regulated in Australia and blasting with silica sand is prohibited or tightly restricted; garnet, copper slag and crushed glass are the usual substitutes, used in an enclosed booth with extraction, media recovery and respiratory protection.
Which materials it suits
Blasting works on any cementitious panel: sprayed and premix GRC, precast concrete, ultra high performance concrete and fibre cement. On glass fibre reinforced concrete the surface paste layer of a sprayed skin is thin and a fibre-rich zone sits below it, so a light or medium blast stays in the paste and fine sand while a heavy blast exposes glass bundles as pale fuzzy patches that look like a defect because they are one. Agree a maximum depth with the manufacturer and keep heavy blasting for thicker precast sections.
Blasting lightens the colour: cutting the face removes the dense, darker surface skin and opens a rough surface that scatters light, so a blasted panel reads one or two shades paler and flatter than the same mix off-form and pigmented mixes lose apparent saturation. Blasting is not used on fibre reinforced polymer facade panels, where the face is a gel coat over a laminate; a matte FRP surface comes from the mould face, the gel coat or a factory coating.
Sandblasted compared with acid etched
- Mechanism: blasting is abrasive impact, etching is acid dissolving the calcium compounds in the paste
- Depth: blasting runs from a few tenths of a millimetre to several millimetres, etching is limited to roughly 0.1 to 0.5 mm
- Read: blasting gives an open, granular, slightly pitted face, etching a fine even sandy face
- Detail: etching preserves fine mould relief and sharp arrises, blasting softens them
- Consistency: etching is more repeatable over large areas because the chemistry self-limits
- Waste: blasting produces spent media and dust, etching acidic rinse water to neutralise
Cost, programme and behaviour in service
Blasting is the least expensive of the paste-removal finishes and the least disruptive to production, since it needs no retarder, no washing window and no fixed timing relative to demoulding and can be scheduled days or weeks after casting. It is also the most forgiving in service. A matte, open surface hides minor colour variation between batches, small repairs and the hairline crazing that can appear in a cement face, which is why blasting is sometimes chosen to level out a run that would show variation if left off-form. The cost is that the open surface holds dirt and is harder to clean, so at ground level an anti-graffiti coating or a hydrophobic impregnation is often added.
How it is specified and sampled
Specify the medium and grade, the nominal depth of removal in millimetres, the maximum permitted depth to protect the skin, and whether blasting is by hand nozzle or mechanised. Require arrises, reveals and returns to be blasted to the same depth as the field of the panel, since those are the areas where hand work drifts.
Then sample. A control panel of at least 600 by 600 mm produced by the production method, and a full-size first-off panel viewed vertically outdoors at the agreed viewing distance, are the minimum. Present the control as two or three pieces showing the acceptable extremes of depth and colour rather than a single ideal piece, and keep matched copies at the plant and on site. No blast setting is meaningful on paper; it is meaningful on a panel.
Standards and references
- AS 1627.4 Metal finishing, preparation and pretreatment of surfaces, abrasive blast cleaning
- Safe Work Australia Workplace Exposure Standard for respirable crystalline silica; abrasive blasting with silica sand is prohibited or restricted
- AS/NZS 4361.1 Guide to hazardous paint management (blasting containment and worker protection)
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (finishes and sampling)
- PCI MNL-117 Manual for Quality Control for Plant Produced Architectural Precast Concrete
- AS 3610.1 Formwork for concrete, surface finish classes
Related terms
Questions specifiers ask
What is the difference between sandblasted and acid etched concrete?
Both remove surface cement paste but by different means and to different depths. Blasting abrades it mechanically and can go from a few tenths of a millimetre to several millimetres, giving an open granular face. Acid etching dissolves it chemically, is limited to roughly 0.1 to 0.5 mm, and keeps crisp edges and fine relief.
Can GRC be sandblasted?
Yes, at light to medium depths. A sprayed skin has a thin surface paste layer over a fibre-rich zone, so a heavy blast exposes glass fibre bundles as pale fuzzy patches. Agree a maximum blast depth with the manufacturer, typically well under a millimetre on a thin skin.
Is real sand used?
Not in professional practice. Respirable crystalline silica is heavily regulated in Australia and blasting with silica sand is prohibited or restricted. Garnet, copper slag and crushed recycled glass are the usual media, used in an enclosed booth with extraction and recovery.
Does sandblasting change the colour?
Yes. Removing the dense surface skin and roughening the face makes it scatter light, so a blasted panel reads one or two shades paler and flatter than the same mix off-form. Pigmented mixes lose apparent saturation, which should be allowed for when the dose is set.
How deep can a blast go?
As deep as you let it, which is why a maximum must be specified. Light is 0.3 to 0.5 mm, medium 0.5 to 1.5 mm, and beyond roughly 1.5 mm on precast it becomes an exposed aggregate finish. On a thin GRC skin the ceiling is much lower.
Need a matte, uniform face across a large panel run? We blast to an agreed depth from a signed control sample and hold it across the whole facade. Discuss a blasted finish
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.