Fibre Reinforced Facade Encyclopedia: Finishes
An off-form finish is a surface taken directly from the face of the mould, with no grinding, rendering, coating or applied texture afterwards. Everything the mould face has, the panel inherits: its material, its sheen, its joints, its screw heads and its previous repairs. It is the cheapest finish to specify and among the hardest to deliver consistently, because the quality is decided in the mould shop before any material is placed.
A steel mould gives a dark, closed, glossy surface. The same mix cast against plywood gives a lighter, matt surface carrying the grain of the sheet. Neither is a defect. They are two different finishes from one material.
At a glance
| Property | Typical value | Note |
|---|---|---|
| AS 3610 Class 1 | Highest class, viewed from 1 to 2 m | Rarely justified over large areas; costly to hold |
| AS 3610 Class 2 | The normal architectural standard for concrete on view | The realistic target for off-form GRC panels |
| Blowhole limits | set by the class table as a maximum diameter and a percentage of a nominated area | |
| Colour variation | judged against an approved sample panel, not an absolute value | The mock-up is the contract benchmark |
| GRC skin | 10 to 20 mm, mould face is the finished face | Ribs and frames are formed on the back |
| FRP gel coat | 0.4 to 0.6 mm sprayed onto the mould face | The finish is formed, not painted on later |
The mould face is the finish
Off-form work inverts the normal relationship between money and appearance. There is no finishing trade to correct anything afterwards, so the cost and the risk sit in the mould. A steel or GRP mould gives a hard, closed, slightly glossy surface and holds it for many casts. A CNC machined polyurethane or MDF board gives a fine matt surface suited to complex geometry but wears faster. Film faced plywood carries the character of the sheet, timber boards give a board mark, and rubber or silicone liners give a repeatable texture.
The release agent is part of the finish, not an incidental: too much retards the surface and shifts the colour, too little tears the face at demoulding. The mould material also affects colour through absorbency, because an absorbent face draws water from the surface layer and lightens it. Mould making and CNC formwork cover how these moulds are produced.
Joint marks, form lines and fixings
A mould larger than a sheet of material has joints, and every joint prints. So does every screw, every patch, every change of mould material, and every place a rib or a return meets the face. These marks cannot be removed from an off-form surface, so the only question is whether they were designed.
The discipline is to set out the form lines on the elevation before the mould is built, so joints land on panel edges, on a rebate or on a deliberate grid and repeat across the facade. The same logic applies to the draft angle, the edge return and the position of any split in the mould. Specifiers who nominate the form line layout get a coherent elevation.
AS 3610 classes and what they mean
AS 3610 is the Australian formwork standard, and its surface finish classes are the common language for off-form appearance even where the element is a factory made panel rather than site cast concrete. Class 1 is the highest, defined for close viewing and tight on blowholes, colour variation, alignment and steps. Class 2 is the normal architectural class for concrete on view, and Classes 3 to 5 run from structural work on view down to concealed work.
Blowholes and colour variation
Blowholes are air trapped against the mould face. They come from the mix, the release agent, the compaction and the mould material, and on a vertical or steeply raked face they concentrate near the top. In sprayed GRC they are controlled by roller compaction against the face, in premix by vibration and mix design, and in FRP by how the gel coat is applied and the first laminate layer is rolled out.
Colour variation in a cementitious face has more causes than any other defect: cement batch, pigment batch, water content, mould absorbency, curing, age at demoulding and the weather on the day. The controls are mundane and effective. Use one cement batch and one pigment batch for a visible elevation, hold the mix constant, demould at the same age, cure identically, and number the panels so variation can be distributed rather than concentrated. An FRP face avoids most of this, because the colour sits in the gel coat.
What to specify
- The finish class and the viewing distance, and whether it applies to the whole elevation or to defined zones
- An approved sample panel or mock-up, retained on site, as the acceptance benchmark
- The mould material, and the maximum number of casts before the face is refurbished
- The form line and mould joint layout on the elevation, agreed at shop drawing stage
- Blowhole and colour criteria, by reference to the class tables and the approved sample
- The repair protocol, and whether any coating or sealer is permitted, since these change the sheen
Surface finish covers the alternatives, including acid etched, sandblasted, polished and exposed aggregate faces, all applied after demoulding and therefore not off-form finishes. Mock-up and tolerances cover approval and dimensional control.
Standards and references
- AS 3610 Formwork for concrete, and AS 3610.1 specification and documentation, including surface finish classes
- Concrete Institute of Australia, Recommended Practice, Off-form concrete finishes
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products, surface finish and tolerances
- PCI MNL-117 Manual for Quality Control for Plants and Production of Architectural Precast Concrete
Where this matters on a ShapeShift project
One Sydney Harbour, Barangaroo: Moulded panels where the visible face comes straight off the mould, so the mould layout, the form lines and the repeatability of the face across many casts are the quality control task.
Related terms
Questions specifiers ask
What is an off-form finish?
A surface taken directly from the mould face with no further treatment. The mould material, its joints, its fixings and its wear all show on the finished panel.
Which AS 3610 class should be specified?
Class 2 for architectural work in most cases. Class 1 is achievable on small controlled elements but is difficult and expensive to hold across a full elevation.
Can blowholes be eliminated?
Not entirely. They are controlled by mix, release agent, compaction and mould material, and judged against the class limits and the approved sample panel.
Why do panels vary in colour?
Cement and pigment batch, water content, mould absorbency, curing and age at demoulding all shift the shade. One batch, one regime and consistent demoulding are the controls.
Setting a finish standard for a moulded facade? ShapeShift produces sample panels and mock-ups so the benchmark is agreed before the moulds are built. Talk to ShapeShift about finishes
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.