Fosroc Nitomortar FC(B)
Two-component, solvent-free epoxy fairing coat and blow-hole filler for concrete surface imperfections from 3 mm up to 5 mm — applied without primer and ready to overcoat.
What is it?
A two-component, solvent-free, thixotropic epoxy resin compound (base plus hardener with graded fillers and thixotropic agents) formulated to fill blow holes and surface irregularities in concrete prior to protective or decorative overcoating.
Where is it used?
On formed and exposed concrete in water-retaining structures, effluent and sewage treatment works, industrial floors, basements, bridge and infrastructure elements, and any substrate being prepared for an epoxy or protective coating system.
Why use it?
It produces a smooth, pinhole-free, chemically resistant base layer in a single thicker pass (up to 5 mm) without a separate primer, reducing the number of subsequent coats needed and improving the finish and adhesion of the topcoat.
How is it applied?
Stir base and hardener separately, combine the full pack at a 1:1 ratio, mix to a uniform light-grey paste, then apply to clean dry concrete by trowel, scraper or filling knife at 3 to 5 mm; allow about 24 hours cure before overcoating.
About Fosroc Nitomortar FC(B)
Fosroc Nitomortar FC(B) is a purpose-made surface-preparation mortar in Fosroc’s wider Nitomortar concrete-repair family. It is a two-component, 100% solids, solvent-free epoxy compound: a light-grey base and a black hardener, combined with graded fillers and thixotropic agents so the mixed material holds on vertical and overhead faces without slumping. The contrasting base and hardener colours are deliberate, giving the applicator a clear visual check that mixing is complete before the paste turns a uniform grey. The “(B)” designation distinguishes it from the thinner Nitomortar FC: where standard FC is intended for feather-edge work up to 3 mm, FC(B) is the heavier-build version for blow holes and irregularities that need a thicker fill, in the band above 3 mm and up to roughly 5 mm in a single application. Because it is solvent-free and applied directly to concrete without a primer, it is well suited to enclosed and water-retaining environments where solvent emissions and extra coats are unwelcome. The cured surface is dense, smooth and chemically resistant, and is designed to be overcoated with Fosroc’s epoxy protective and decorative systems such as the Nitocote range, making FC(B) a true preparation product rather than a final wearing surface. For Indian contractors and applicators, Space Arc Engineering supplies it as part of a complete blow-hole-filling and coating package, typically alongside the relevant surface primer and topcoat for a given exposure class.
The technical insight
The engineering value of Nitomortar FC(B) sits in its thixotropy combined with a high filler loading. Plain epoxy fairing coats are limited to very thin films because unfilled resin slumps and exotherms when built up; FC(B) uses graded mineral fillers and rheology modifiers to carry a 3 to 5 mm build in one pass on vertical and soffit surfaces while keeping shrinkage low and the cured matrix dense (fresh wet density of the order of 1700 kg/m3 when fully compacted). The mix is supplied at a fixed 1:1 ratio so site batching errors are minimised, and the contrasting base/hardener colours act as a built-in mixing-quality control. Mechanically the cured compound is strong for a fairing material — on the order of 70 N/mm2 compressive strength at 7 days — which means the fill is never the weak link beneath the topcoat. The trade-off is a temperature-sensitive working window: pot life runs to about 4 hours at 20°C but drops to roughly 1.5 hours at 35°C, so in Indian summer conditions packs should be part-planned to the work face and used promptly. Full chemical resistance and bond develop after curing, after which the lightly abraded surface accepts epoxy overcoats reliably.
Where it is used
- Filling blow holes, bug holes and surface voids in formed concrete before protective coating
- Faring out honeycombing and minor irregularities greater than 3 mm on walls and soffits
- Surface preparation of concrete in water-retaining and effluent/sewage treatment structures
- Preparing substrates in basements, tunnels and below-grade structures ahead of waterproof epoxy systems
- Smoothing industrial and warehouse floor surfaces prior to epoxy floor coatings
- Bridge decks, piers and infrastructure repair where a chemically resistant base coat is required under a topcoat
Key advantages
- Solvent-free and 100% solids — suitable for enclosed, water-retaining and low-odour environments
- Applied directly to concrete with no separate primer, saving a coat and labour
- Single-pass build from above 3 mm up to about 5 mm thanks to non-slump thixotropic body
- Contrasting light-grey base and black hardener give a visual confirmation of thorough mixing
- Fixed 1:1 mix ratio reduces on-site batching error
- High compressive strength (around 70 N/mm2 at 7 days) so the fill does not undercut the topcoat
- Excellent chemical resistance and strong adhesion to concrete, extending coating-system durability
- Reduces the number of subsequent coatings needed for a smooth, pinhole-free finish
Limitations & considerations
- A surface-preparation / fairing product, not a structural repair mortar or a finished wearing surface
- Build is limited to roughly 5 mm per application — deeper section loss needs a structural Renderoc/Nitomortar repair mortar
- Pot life shortens sharply in heat (about 1.5 hours at 35°C), demanding fast placement in Indian summer
- Part packs must not be used and part-mixing is not recommended — the full 1:1 pack should be mixed at once
- Requires a clean, dry, laitance-free substrate prepared by gritblasting or mechanical means for proper bond
- Needs roughly 24 hours cure and light abrasion before overcoating; not an instant-overcoat product
- Epoxy chemistry means application is temperature-sensitive and not suited to wet or actively damp surfaces during cure
Selection & best practice
Prepare the concrete to a sound, clean, dry, laitance-free surface by gritblasting or mechanical abrasion before any material is opened. Condition packs out of direct sun, then stir the base and hardener separately to disperse settlement, empty the entire hardener into the base and power-mix with a slow-speed (400 to 500 rpm) heavy-duty drill and Fosroc paddle until a uniform grey paste is achieved — never part-mix or use part packs. Apply at 3 to 5 mm by trowel, scraper or filling knife, working only as much material as can be placed within the pot life (plan to the heat — roughly 1.5 hours at 35°C). Allow about 24 hours cure, then lightly abrade and clean the faired surface before applying the chosen Fosroc epoxy topcoat. Clean tools immediately with the recommended Fosroc cleaning solvent before the resin sets.
Specifications
| Chemical type / system | Two-component, solvent-free, 100% solids thixotropic epoxy resin compound with graded fillers |
| Components / colour | Base (light grey) + Hardener (black); mixes to uniform grey |
| Mixing ratio | 1:1 by volume (full pack) |
| Application thickness | Above 3 mm up to approximately 5 mm per application |
| Compressive strength | ~70 N/mm² @ 7 days |
| Fresh wet density | ~1700 kg/m³ (fully compacted) |
| Pot life | ~4 hours @ 20°C; ~1.5 hours @ 35°C |
| Overcoating / initial cure | Minimum ~24 hours before overcoating with epoxy systems |
| Coverage | ~0.33 m²/litre at 3 mm thickness (varies with profile and thickness) |
| Primer | Not required — applied directly to prepared concrete |
| Chemical resistance | Resistant to oils, fats, petrol, salt and sugar solutions, bleach, dilute mineral acids, alkalis and various solvents |
| Pack sizes | 1 litre and 4 litre packs |
| Substrate | Clean, dry, laitance-free concrete |
Industry use cases
Water & wastewater infrastructure
Filling blow holes in walls and floors of water-retaining tanks, STPs and effluent works before a chemically resistant epoxy protective lining is applied — the solvent-free formulation suits enclosed wet environments.
Industrial flooring
Fairing out voids and irregularities on warehouse and factory concrete floors so that an epoxy floor coating goes down pinhole-free and with strong adhesion.
Bridges & transport infrastructure
Smoothing surface imperfections on piers, decks and abutments under protective coating systems as part of durability and rehabilitation works.
Basements & below-grade structures
Preparing concrete in basements, tunnels and retaining walls ahead of waterproof epoxy systems, where the no-primer, single-pass build speeds up the programme.
Commercial & precast finishing
Filling bug holes and honeycombing on exposed and precast concrete facades before decorative or protective overcoating for a uniform finish.
Frequently asked questions
Is Nitomortar FC(B) a structural repair mortar?
How is FC(B) different from standard Nitomortar FC?
Does it need a primer?
What is the mixing ratio and pot life?
What can I overcoat it with, and when?
How much do I need and what pack sizes are available?
How we position Fosroc Nitomortar FC(B)
Within Fosroc’s system, Nitomortar FC(B) is the heavier-build sibling of Nitomortar FC and sits upstream of the protective topcoats it prepares for, most commonly the Nitocote epoxy coatings; for deeper structural reinstatement it is paired with, not substituted by, the Renderoc and Nitomortar structural repair mortars, and on coating projects it is often used alongside a Nitoprime/Nitocote primer and the relevant epoxy or polyurethane finish. As an authorised Fosroc distributor and applicator, Space Arc Engineering specifies FC(B) when the priority is a verified, single-source coating system with full TDS backing. Credible alternatives exist in the Indian market and are worth naming honestly: Sika offers epoxy levelling and blow-hole fillers within its Sikadur and Sikagard surface-preparation lines; MBCC/Master Builders Solutions (the former BASF construction chemicals business, “MBS”) has comparable epoxy fairing and patching products in its MasterSeal and MasterEmaco ranges; Dr. Fixit (Pidilite) and STP (Shalimar/STP Limited) supply epoxy putties and surface fillers that compete on price and local availability, though their published technical data and applicator support vary by product. The right choice usually comes down to compatibility with the chosen topcoat system, documented chemical-resistance data for the exposure, and the availability of trained applicators — areas where staying within a single Fosroc system, supplied and applied by Space Arc Engineering, reduces interface risk on water-retaining and industrial projects.
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