Fibre Reinforced Facade Encyclopedia: Finishes
Surface finish is the visible texture, colour and gloss of a facade panel. On moulded materials such as glass fibre reinforced concrete (GRC, GFRC), fibre reinforced polymer and precast, the finish is produced by the mould face and the mix rather than by something applied afterwards. The mould is the finish. That single fact explains why finish decisions have to be made before tooling is cut, not during the coating package.
Two panels from the same GRC mix, one cast against a polished steel mould and one against a rubber form liner, then one of each acid etched, give four different finishes from one material and one colour. Nothing was painted.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Off-form | As cast against the mould face | Finish quality equals mould quality |
| Form liner texture | Relief typically 3 to 25 mm deep | Rubber, plastic or CNC-cut liner in the mould |
| Exposed aggregate | Aggregate 3 to 20 mm exposed | Surface retarder and wash, or abrasive removal |
| Acid etch | Removes about 0.5 to 1.5 mm of paste | Fine uniform matt; softens colour |
| Sandblast | Light, medium or heavy; 1 to 3 mm | Coarser and more forgiving than etch |
| Polished or honed | Grit sequence to 400 plus | Reveals aggregate; shows any casting defect |
| Gel coat (FRP) | 0.4 to 0.6 mm pigmented layer | Moulded in; is the surface, not a coating on it |
| PVDF coating | 25 to 40 microns dry film | Applied; 20 to 30 year colour warranty typical |
| Colour tolerance | Assess at 3 m in diffuse daylight | Agree the method with the mock-up, not on site later |
| Efflorescence | Cementitious panels only | Reduces over 6 to 18 months of weathering |
The finish comes from the mould and the mix
A cast or laminated panel takes the negative of whatever it was formed against, down to the grain of the plywood and the scratch left by a trowel on a steel bed. Against polished steel or GRP a cementitious panel comes out dense, closed and glossy; against plywood it carries the board’s grain and joints; against a form liner it carries whatever relief was in the liner. Nothing has been added to the panel.
The mix contributes the other half. In cementitious materials the colour comes from the cement, grey, off-white or white; from the sand and any decorative aggregate; and from oxide pigments dosed against cement weight. In a polymer composite it is in the pigmented gel coat laid into the mould before the laminate. Because the colour is integral rather than painted, a chip does not expose a different colour underneath, which is the main durability advantage of a moulded finish over a coated one.
This is why the finish must be resolved before the moulds are made. Going from off-form to a textured liner after tooling is committed means new tooling. Going from acid etch to sandblast after casting is usually possible, because both are post-cure treatments; changing the aggregate is not.
The finishes in use
- Off-form: the as-cast surface, taken directly from the mould. Highest gloss and lowest cost, and the least forgiving, because every mould defect reads through.
- Textured: a form liner, CNC-cut mould or applied pattern gives ribs, reveals, board marking, stone effect or a bespoke relief. Texture hides tolerance and colour variation better than any other finish.
- Exposed aggregate: a retarder on the mould face delays the set of the outer paste, which is washed off after demoulding to reveal the aggregate. Retarder grade sets the depth.
- Acid etched: dilute acid dissolves the surface paste, giving a fine uniform matt with the sand just visible. Lightens and flattens the colour.
- Sandblasted: abrasive removes paste mechanically at light, medium or heavy intensity. Coarser than etch and more tolerant of variation.
- Polished or honed: progressive grits give a smooth, closed, reflective surface that exposes the aggregate fully. Unforgiving; every void and colour shift shows.
- Painted and coated: an applied system over the cured panel. PVDF gives the best exterior colour retention; anti-graffiti coatings usually go over one of these.
- Gel coat: the polymer composite equivalent of off-form, a pigmented resin layer laid into the mould first so it becomes the outer surface of the laminate.
Colour consistency across batches
Colour variation is the most common finish dispute on a facade and it is almost never caused by the pigment. It is caused by everything else moving: a different cement delivery, a new sand stockpile, a change in water content, a longer or shorter cure, a mould that has aged and become more absorbent, or panels cast in January and February in different ambient conditions.
Controlling it means reserving one batch of cement and one aggregate stockpile for the whole facade, fixing the water to cement ratio and mixing time, holding the cure constant, rotating moulds so no panel is cast against a much older face than its neighbours, and casting each elevation close together in time. On polymer composites the equivalents are a single gel coat batch, controlled film thickness and controlled shop temperature.
Efflorescence and weathering
Efflorescence is a white bloom on cementitious panels caused by soluble calcium hydroxide migrating to the surface with moisture and carbonating there. It is cosmetic, it is worse on panels cured or stored wet, and it is far more visible on textured and dark-pigmented finishes. Primary efflorescence usually fades over 6 to 18 months of weathering. Reduce it at source with a low water to cement ratio, pozzolanic additions, a controlled cure and dry storage. It can be removed with a proprietary cleaner or dilute acid wash, which lightens the finish slightly and should be trialled on a spare panel first.
Weathering is the longer-term version of the same problem. Runoff concentrating at a sill, nib or projection deposits dirt in streaks. Drips, throats, recessed joints and a slight fall on horizontal surfaces spread the runoff so the facade ages evenly. A hydrophobic silane or siloxane treatment reduces water uptake and dirt retention without changing appearance, on a roughly 10 year maintenance cycle.
Repairability
A moulded integral finish is repairable in a way that a coated one is not entirely, and vice versa. A chip in an off-form, etched or sandblasted cementitious panel exposes the same material and colour underneath, so a patch made with the identical mix and treated to match disappears at viewing distance once weathered. A polished finish is far harder to match, because the repair reads differently in reflected light. Exposed aggregate is intermediate, since the aggregate has to be hand-placed into the patch.
A gel coat finish on FRP is repaired with matching gel coat, cured, cut back and polished, and is the easiest of all to make invisible, which is one reason polymer composites tolerate site damage well. An applied paint or PVDF system can be touched up, but the touch-up uses a different application method to the factory coating and is usually visible in raking light until it weathers.
Keep spare panels and a documented mix or colour record for every facade. Ten years on, the cement, the sand source and the pigment batch will all have changed, and the record is the only way to get close.
Standards and references
- AS 3610 Formwork for concrete, surface finish classes and appearance requirements
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products (finish and appearance)
- PCI MNL-117 Manual for Quality Control for Plants and Production of Architectural Precast Concrete
- AAMA 2605 (superior performing organic coatings, PVDF) and AAMA 2604
- ASTM D523 (specular gloss), ASTM D2244 (colour difference), ISO 7724
- ASTM C1364 (architectural cast stone) and ASTM D4541 (coating adhesion by pull-off)
Related terms
Questions specifiers ask
Is the finish on a GRC or precast panel a coating?
Usually not. The finish comes from the mould face and the mix, then optionally from a post-cure treatment such as acid etch or sandblast. Coatings are used where a colour or gloss cannot be achieved in the material itself, or where anti-graffiti performance is required.
Which finish hides colour variation best?
Texture. A form liner, deep relief or heavy sandblast breaks up reflected light and disguises both colour variation and small dimensional tolerance. Polished and off-form finishes are the least forgiving.
How do you stop efflorescence on cementitious panels?
Reduce the water to cement ratio, use pozzolanic additions, cure in a controlled way and keep panels dry in storage. Primary efflorescence that does appear generally fades over 6 to 18 months of weathering, and can be cleaned with a proprietary cleaner, trialled first on a spare panel.
Can a damaged panel be repaired on site?
Usually. Cementitious panels are patched with the same mix and the surface treatment matched; gel coated polymer composite panels are repaired with matching gel coat and polished back. Polished concrete and applied paint systems are the hardest to make invisible.
When does the finish have to be decided?
Before the moulds are made. Texture and relief are cut into the tooling, so changing them later means new tooling. Post-cure treatments such as etching and blasting can be varied later; aggregate and colour cannot.
How should colour be assessed?
Against approved light and dark range samples, viewed at about 3 metres in diffuse daylight on the built elevation. Fix the method in the specification and confirm it on the visual mock-up before production starts.
ShapeShift produces finish samples and full visual mock-ups from the project tooling before production, so the finish is agreed on the real material. Request a finish sample
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.