Fosroc Renderoc FC
A single-component, polymer-modified fairing coat that blinds blowholes, honeycombing and surface voids up to 3 mm, leaving concrete fair-faced and ready to coat.
What is it?
A factory pre-blended, polymer-modified cementitious fairing coat that fills surface imperfections in concrete and masonry up to 3 mm deep, needing only clean water on site.
Where is it used?
Vertical and overhead concrete and masonry surfaces on bridges, flyovers, water-retaining structures, facades and precast elements where a smooth, fair-faced finish is needed before coating.
Why use it?
It cosmetically levels blowholes, tie-bolt marks and honeycombing so anti-carbonation and protective coatings sit on a continuous, defect-free surface, and it bonds without a separate primer.
How is it applied?
Clean and pre-soak the substrate, mix one bag with the specified water, then trowel from a feather-edge up to 3 mm and close to a smooth finish; deeper voids are filled first as a prior operation.
About Fosroc Renderoc FC
Renderoc FC is Fosroc’s fairing coat within the wider Renderoc concrete repair system, and it does a job that is easy to underrate on a repair contract. After a structural patch has been placed, or after formwork has been stripped from fresh concrete, the surface is rarely uniform: blowholes, tie-bolt holes, minor honeycombing and pattern lines all remain. If a protective or decorative coating is applied straight over that, the defects telegraph through and become weak points for water and carbon dioxide ingress. Renderoc FC exists to blind those imperfections and hand over a continuous, fair-faced surface. It is supplied as a ready-blended bag of cements, graded fine aggregates, special fillers and chemical additives, so the applicator only adds clean water on site and mixing variation is designed out.
It is important to be clear about what this material is and is not. Renderoc FC is a thin fairing coat, not a structural or high-build repair mortar. Its working range is a feather-edge up to 3 mm per pass, and it is meant for surfaces that are not trafficked and are subsequently coated or left exposed. Anything deeper than 3 mm, or any repair carrying load, belongs to the load-bearing Renderoc mortars, with Renderoc FC applied afterwards as the finishing skin. Used that way it is genuinely low-fuss: under normal conditions it needs no independent primer and no curing membrane, it contains no chloride admixtures, and it is fully compatible with the rest of the Renderoc range and Fosroc Dekguard coatings.
The technical insight
The performance of a fairing coat is really a question of bond and permeability at very thin section, and that is where the polymer modification earns its place. A conventional sand-cement skim at 2-3 mm has almost no reservoir of water and shrinks as it dries, so it tends to debond or micro-crack over a dense concrete substrate. The polymer in Renderoc FC forms a continuous film through the hardened matrix that improves adhesion to the parent concrete and resists the passage of water, carbon dioxide and chloride ions, so the thin coat behaves as a durable barrier rather than a sacrificial cosmetic layer. The graded fine aggregate and fillers keep the water demand low (a water:powder ratio of about 0.3 by weight, or 1:3 by volume), which is what lets the material be worked from a feather-edge and closed to a smooth finish without excessive bleed or slump. Because it is engineered to have thermal compatibility with concrete, the coat expands and contracts broadly in step with the substrate through daily and seasonal cycles, avoiding the differential movement that would otherwise pop a thin skim off. The trade-off is deliberate: the same fine, low-build formulation that gives a smooth finish is why the material is capped at 3 mm and is not intended to rebuild section.
Where it is used
- Blinding blowholes and air voids on stripped in-situ concrete before an anti-carbonation coating
- Rectifying honeycombing and surface porosity on bridge piers, abutments and flyover columns
- Filling tie-bolt holes and form marks on retaining walls and water-retaining structures
- Producing a fair-faced finish on precast panels and architectural concrete
- Fairing over completed Renderoc structural patch repairs to unify the surface
- Smoothing minor surface imperfections on masonry ahead of decorative coatings
- Skim levelling of soffits and vertical faces on parking structures prior to protection
- Preparing facades and exposed concrete for the Dekguard protective coating system
Key advantages
- Single-component and pre-blended, so only clean water is added on site and batch-to-batch variation is eliminated
- Bonds strongly to the substrate without an independent primer or bonding slurry under normal conditions
- Polymer modification gives low permeability to water, carbon dioxide and chloride ions in a thin coat
- Thermally compatible with concrete, reducing the risk of differential movement and debonding
- Can be left exposed or overcoated with protective and decorative finishes such as Dekguard
- Contains no chloride admixtures and is classed RCS (respirable crystalline silica) hazard-free
- Fully compatible with the rest of the Renderoc repair range, allowing a single-system specification
- Low water demand allows a workable, feather-edge finish and a genuinely smooth closed surface
Limitations & considerations
- Strictly a thin fairing coat: maximum applied thickness is 3 mm per pass, so it cannot rebuild section or carry load
- Not suitable for surfaces subject to vehicle or foot traffic
- Should not be applied when substrate or air temperature is 5 C and falling
- Vulnerable before final set: exposure to rain or moving water can severely reduce hardened performance
- Voids deeper than 3 mm (up to 30 mm and no wider than about 25 mm) must be filled separately as a prior operation
- Short working life of roughly 20 minutes at 20 C means small, promptly-used mixes are essential
Selection & best practice
Start with the substrate, because the coat is only as good as what it grips. Remove all dust, laitance, curing compound, oil, paint and algae, and lightly scabble or grit-blast to expose the fine aggregate, then blow clean with oil-free compressed air. Deal with deep defects first: blowholes and voids from 3 mm up to about 30 mm deep and no more than 25 mm across should be filled with Renderoc FC and allowed to set before the fairing pass. Soak the prepared area thoroughly with clean water and remove standing water just before application. Always add powder to water, not the reverse, using a forced-action or paddle mixer (free-fall drum mixers are unsuitable) with roughly 5.45-6.35 litres of water per 20 kg bag, and never exceed the top water figure. Mix only what can be used inside the 20-minute working life. Trowel from a feather-edge up to 3 mm, work it minimally, let it partly set, then close to a smooth finish; a light flick of water before final trowelling helps where a very fine finish is wanted. In hot or windy conditions cure with Nitobond AR or polythene as trowelling progresses; do not apply with rain imminent unless sheltered. Allow a minimum of 48 hours before overcoating with a protective or decorative system.
Specifications
| Material type | Single-component, polymer-modified cementitious fairing coat |
| Maximum applied thickness | Feather-edge up to 3 mm per pass |
| Working life | Approx. 20 minutes at 20 C |
| Setting time (BS 4550) | 30 minutes to 1 hour at 20 C |
| Water demand | 5.45-6.35 litres per 20 kg bag (w:p approx. 0.3 by weight) |
| Coverage / yield | Approx. 12.0 litres per 20 kg bag (about 4.0 m2 at 3 mm) |
| Fresh wet density | Approx. 2000 kg/m3 |
| Substrate compatibility | Non-structural concrete of 20-35 MPa compressive strength |
| Pack size | 20 kg bag |
Industry use cases
Bridges & Flyovers
Fairing over pier, abutment and soffit repairs to remove blowholes and honeycombing before an anti-carbonation coating protects the structure from chloride and CO2 ingress.
Water-Retaining Structures
Blinding tie-bolt holes and form marks on tank and reservoir walls so the surface is continuous and defect-free ahead of a protective coating.
Precast & Architectural Concrete
Restoring a uniform fair-faced appearance on panels and exposed concrete elements where the finish is visible or lightly decorated.
Parking & Industrial Structures
Smoothing vertical faces and soffits on car parks and plant buildings, applied over completed Renderoc structural repairs to unify the surface before protection.
Frequently asked questions
Is Renderoc FC a structural repair mortar?
Do I need a separate primer or bonding agent before applying it?
How much area does one bag cover?
Can I fill a 10 mm deep blowhole in one go?
How long before I can apply a protective coating over it?
Can it be used on floors or trafficked areas?
How we position Fosroc Renderoc FC
Renderoc FC is the finishing member of the Fosroc Renderoc concrete-repair family, so it is best specified alongside the products that do the heavy lifting. For the structural repair itself, pair it with Renderoc HB40 for high-build vertical and overhead patching, or Renderoc RG where a rapid strength gain is needed; Renderoc FC then fairs over the completed patch and the surrounding concrete to give one continuous, coatable surface. Once faired, protect the structure with a Fosroc Dekguard anti-carbonation coating to lock out chlorides and CO2. Where a project team is standardised on other systems we distribute, the closest counterparts are Sika’s MonoTop fairing and repair range and Dr. Fixit’s Krishna concrete-repair mortars, though for compatibility we recommend keeping the repair and fairing within one manufacturer’s system. Speak to our technical team in Delhi NCR to match the fairing coat to the structural mortar already specified.
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