Decorative mineral finishes are specified badly more often than almost anything else on a schedule of finishes and the reason is structural rather than careless: there is no British Standard for them, manufacturers' literature is written for sales rather than specification and the trade uses the same words for materials that are not the same. This is what we would want in a specification if we were reading it rather than writing it.
Name the binder, not the look
"Polished plaster" describes an appearance. It covers authentic lime Stucco Veneziano and acrylic imitations costing a fraction as much and both can be tendered against the same clause entirely legitimately. If the specification does not name the binder, the contractor is entitled to price the cheapest thing that matches the description and the client meets the difference at handover.
Write lime-based, polymer-modified cementitious or clay. That single word does more work than a paragraph of adjectives.
Specify the coat count and the compression
The largest cost variable within any of these finishes is days of skilled labour and that is set by coat count and how hard the surface is worked. A four-coat honed Marmorino and a seven-coat burnished stucco use comparable materials and can differ by a factor of two in price. Both will be offered as "Venetian plaster".
State the number of finish coats and the target sheen - matt, satin, or burnished - and require a sample panel against which it will be judged.
Build-up depths
| System | Nominal build | Note |
|---|---|---|
| Venetian plaster / Marmorino | < 1 mm | Over prepared substrate; no levelling capacity |
| Microcement, walls | 2 mm | Base + finish coats |
| Microcement, floors | 3 mm | Includes mesh reinforcement |
| Clay plaster | 2-3 mm | Base with fibre + finish |
| Lime plaster, three-coat | 15-25 mm | Traditional work on solid walls |
The consequential dimensions are usually at thresholds, door undercuts and level-access junctions. Microcement's thinness is frequently the reason it is chosen on a refurbishment and that only holds if the drawings reflect it.
Substrate tolerance is the specification
This is the clause most often missing and its absence is the most common cause of a dispute. None of these finishes has levelling capacity. A burnished lime surface reads every undulation the light can find and at under a millimetre it cannot correct anything.
Specify the required flatness of the receiving substrate, who is responsible for achieving it and at what point it will be inspected and accepted. Without that, the decorative contractor inherits the plasterer's tolerance and the argument happens on site with a client present.
For each substrate type, the things worth naming:
- Plasterboard. Board thickness, fixing centres, joint treatment and whether a skim is required.
- Sand-cement screed. Cure time and the moisture threshold, measured by instrument.
- Anhydrite screed. Mechanical removal of laitance and the primer regime. Most microcement floor failures we inspect start here.
- Existing tile. Bond integrity to be verified by tap test; joints filled and reinforced.
- Timber and board decks. Deflection limit, board rigidity, fixing centres. Deflection is the enemy.
- Solid masonry, pre-1919. Vapour-open systems only. Do not specify a sealed cementitious finish on a wall with no cavity and no DPC.
Movement joints
Every structural and expansion joint in the substrate must be carried through to the finished surface, because at 3 mm nothing bridges movement. On large open-plan floors this means the joint locations should be a design decision, not a site decision. Set them out at a threshold, a change of use, or under an island and mark them on the finishes drawing before the screed is poured.
Waterproofing in wet areas
State plainly that the finish is not the waterproofing. Specify the waterproofing system, require reinforcing tape at all internal corners, junctions and penetrations and require inspection or test of the membrane before the finish is applied, with a hold point. The reasoning is set out here. A specification that omits the membrane invites a price that omits it too.
Performance clauses worth including
- Slip resistance. For wet-room floors and external surfaces, specify a pendulum test value rather than the word "anti-slip". The additive goes in the seal and the value is achievable and stateable.
- Abrasion. For commercial floors, specify the traffic class and let the seal be selected against it.
- Chemical resistance. Relevant for kitchens, laboratories and pools, where the water treatment determines the seal.
- Fire. Reaction-to-fire classification is available from the manufacturer's data sheet for each system. Mineral finishes generally perform well, but on an escape route it should be stated rather than assumed.
- VOC. Where the project is chasing an environmental standard, clay and lime systems are essentially zero-VOC; polyurethane seals are not and low-emission alternatives exist and should be named.
Programme and cure
These are wet, multi-coat finishes with cure time between coats and cure time is not float. Indicative durations we would work to:
- Microcement bathroom: 5-7 working days, plus seal cure before use.
- Microcement floor: 4-6 working days to light foot traffic, longer to full use.
- Venetian plaster room: dependent on coat count; typically 3-6 days plus panel approval beforehand.
- Lime plaster: carbonation over weeks. Cannot be accelerated, cannot be applied in frost.
These trades belong late in the programme, after wet trades and first fix, in a clean and controlled environment. Specifying a decorative finish to run concurrently with other trades in the same room is specifying damage.
Sample panels and approval
Require a physical sample panel, made in the project's own light, approved in writing before application and retained on site as the benchmark for the works. This protects everyone: the client sees what they are getting, the contractor has a defined target and the argument about whether the finish is "right" has an object to refer to rather than a memory.
Where the scheme is significant, a full mock-up of a return, a corner or a niche is worth the cost - flat panels do not tell you how a finish behaves where it turns.
A clause you can lift
Decorative mineral finish - polished lime plaster
Lime-based decorative plaster comprising slaked lime binder and graded marble aggregate, mineral pigmented. [n] finish coats, compressed to a [matt / satin / burnished] sheen, finished with [natural wax / mineral seal]. Applied by manufacturer-trained applicators to a substrate prepared to [tolerance] and accepted in writing prior to commencement. Sample panel of not less than 1 m² to be prepared in situ and approved in writing before the works proceed; panel to be retained on site as the standard for the works. Acrylic and resin-bound alternatives are not acceptable.
What we will do at design stage
Send us the drawings and we will return system recommendations, build-up depths, budget guidance and lead times, usually within two working days - no charge and no obligation. We will also tell you where we think a specification is heading for trouble, which is generally more useful than the recommendation.
Technical data sheets and substrate guides are here and we are happy to attend a design team meeting in person or remotely. Get in touch.