Fibre Reinforced Facade Encyclopedia: Finishes
A polished concrete finish is produced by grinding the cast surface with a sequence of progressively finer diamond abrasives until the cement paste and fine aggregate are cut back to one flat plane and the pores are closed. Acid etching and sandblasting remove paste to open a surface up; polishing removes paste and then burnishes what is left, so the result is denser and less absorbent than the surface it started from. Nothing is added except, usually, a chemical densifier.
A honed GRC panel typically loses 0.2 to 0.5 mm of thickness. Another 0.5 mm and the grinder is into the fibre-rich layer, and glass bundles show as pale streaks.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Process | Wet or dry diamond grinding | Planetary or single-head machine, factory bench work |
| Grit sequence | Typically 50, 100, 200, 400, 800, 1500 grit | Honed stops near 400; full polish runs to 1500 or 3000 |
| Material removed | 0.2 to 1.0 mm typical | Panel thickness must allow for it |
| Gloss, honed | 10 to 30 gloss units at 60 degrees | Matt to satin, no reflected image |
| Gloss, polished | 50 to 80 gloss units at 60 degrees | Reflective |
| Suitable substrates | GRC, UHPC, precast, terrazzo, GRG | Not FRP; gel coat is buffed, not ground |
| Geometry limit | Flat and single-curved faces only | Corners, returns and relief cannot be reached |
| Programme | Typically 2 to 5 days per panel run | Each grit is a full coverage pass |
| Cost premium | Typically 30 to 60 percent over off-form | Labour, consumables and rework |
How the finish is achieved
Polishing is abrasion in stages. Each grit cuts out the scratch pattern left by the grit before it, so the sequence cannot be short-cut; jumping from 100 to 800 leaves coarse scratches visible under the gloss as a haze. The first pass decides the finish, because it sets how deep the surface is cut and how much aggregate is opened up. Later passes only refine what it exposed.
Around 200 or 400 grit a chemical densifier, usually a lithium or sodium silicate, is applied. It reacts with free lime to fill capillary pores and harden the surface, and grinding after densifying is what produces gloss, because the abrasive burnishes a harder surface rather than tearing it. Without it a panel can be ground smooth but will not hold a shine. The whole operation runs on a factory bench with the panel face up; site polishing of cladding is not done.
Which materials take a polish
The result depends on what the abrasive finds under the paste. Precast and UHPC with graded decorative aggregate polish into a terrazzo-like surface where the stone dominates. Sprayed GRC does not: the matrix is cement and fine silica sand with no coarse aggregate, so polished GRC reads as a dense, fine, slightly mottled stone. That is a legitimate result, but a specifier expecting visible aggregate will be disappointed. Where stone is wanted, it is added in a premix face coat or the panel is made as an exposed aggregate finish instead. Glass fibre reinforced gypsum takes a very fine polish and is used that way on interior feature elements. Fibre reinforced polymer is not polished; gloss on an FRP face comes from the mould and from buffing the gel coat.
Gloss level, colour and geometry
Specifying polished is not specific enough. Honed finishes stop at 200 to 400 grit: flat, matt to satin, forgiving of hand marks. Polished finishes run to 800 grit or finer, reflect a soft image, and are far less forgiving, so gloss should be nominated as a target at 60 degrees with a tolerance band and checked with a meter.
Polishing also darkens. Cutting through the pale laitance exposes the body of the mix, so a polished panel reads darker and more saturated than the off-form panel beside it, and an integrally pigmented mix reads stronger. The geometry limit usually matters more than the cost: a rotating head reaches flat faces and gentle single curvature but not internal corners, reveals, deep relief or tight returns, which have to be hand-ground and never quite match. Panels with texture and relief or double curvature are generally not candidates.
Cost, programme and handling
Polishing is the most expensive common finish because it is sequential labour: six or more full coverage passes plus densifying and cleaning between them. Rework is expensive too, since a burn mark or a scratch from stray grit means going back several grits and working forward again.
Polished panels are vulnerable in transit. Faces need soft interleaving, edges need protection, and panels must be kept dry, because water standing on a polished cementitious face leaves a mineral ring that has to be ground out.
How it is specified and sampled
- Nominate honed or polished and give the final grit, not just the word
- Give a target gloss at 60 degrees with a tolerance, for example 60 plus or minus 10 gloss units
- State the maximum permitted material removal in millimetres to protect the skin
- Require the densifier type to be declared
- Require a control sample of at least 600 by 600 mm from the production mix, and a full-size first-off panel
- State how the face is protected in transport and who cleans it at handover
The control sample is the contract. Sign it, keep matched copies at the plant and on site, and accept production against it at the viewing distance the specification sets rather than at arm’s length. Because polished surfaces vary, present the control as a range showing the acceptable extremes rather than one ideal piece; colour consistency covers how that range is set.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (surface finish and sampling)
- AS 3610.1 Formwork for concrete, surface finish classes (reference for the off-form comparison)
- ASTM D523 Standard Test Method for Specular Gloss (60 degree gloss measurement)
- PCI MNL-117 Manual for Quality Control for Plant Produced Architectural Precast Concrete
- Concrete Polishing Council of the ASCC, gloss and aggregate exposure classification
- AS 1530.1 combustibility remains a property of the panel, not the finish (check the edition of the NCC adopted in your jurisdiction)
Related terms
Questions specifiers ask
Can a GRC panel be polished?
Yes, within limits. A sprayed skin can be honed or polished provided the cut stays shallow, typically under half a millimetre, so the abrasive does not reach the fibre-rich layer below the surface paste. Agree the maximum permitted removal with the manufacturer first.
Will polishing show the aggregate?
Only if there is aggregate to show. Precast and UHPC with graded decorative stone polish into a terrazzo-like surface. Standard sprayed GRC uses fine silica sand with no coarse aggregate, so it polishes to a dense fine-grained surface.
What is the difference between honed and polished?
Grit. Honed stops at 200 to 400 grit and reads matt to satin with no reflected image. Polished continues to 800 grit or finer and reflects a soft image, typically 50 to 80 gloss units at 60 degrees.
Does polishing make the panel more durable?
It makes the surface denser and less absorbent and the densifier hardens the top layer. It does not change the structural properties of the panel and it does not change its fire classification, which is a property of the tested product.
Can polishing be done on site?
Not on cladding. The slurry, water, machine weight and lighting control belong in a factory. Site work is limited to cleaning and at most very local touch-up, which will not match.
How much does a polished finish add?
Typically 30 to 60 percent over off-form, and two to five days on the panel programme, because each grit pass is a full coverage pass and there are six or more of them.
Deciding between a polished, acid etched or sandblasted face? We make sample panels from the production mix so the decision is made on the real material. Talk to us about finish samples
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.