Fosroc Renderoc BS
The wet-on-wet bonding coat that decides whether your concrete repair actually holds — a polymer-modified cementitious slurry brushed between prepared substrate and Renderoc repair mortar.
What is it?
A single-component, polymer-modified cementitious bonding slurry (dry powder gauged with clean water or a Renderoc acrylic liquid to a creamy, brushable consistency).
Where is it used?
On mechanically prepared concrete and exposed reinforcement, immediately before applying Fosroc Renderoc patch-repair mortars — bridges, water-retaining structures, columns, soffits and industrial concrete across Delhi NCR and pan-India.
Why use it?
It creates a chemically compatible cement-to-cement bond, wets out the substrate to stop it drawing water out of the fresh mortar, and lifts the interface bond strength that keeps a repair from delaminating.
How is it applied?
Scrub the freshly mixed slurry into a saturated-surface-dry substrate with a stiff brush at roughly 1.0-1.5 kg/m2 per coat, then place the repair mortar wet-on-wet while the coat is still tacky.
About Fosroc Renderoc BS
Every patch repair lives or dies at the interface between old concrete and new mortar. Fosroc Renderoc BS exists to control that interface. It is a factory-blended, polymer-modified cementitious bonding slurry supplied as a single-component dry powder; on site you gauge it with clean water — or a Renderoc acrylic bonding liquid where a tougher bond line is specified — into a creamy slurry that brushes cleanly into a prepared surface. Because it is a cementitious system rather than a rigid epoxy film, it stays chemically and thermally compatible with both the parent concrete and the Renderoc repair mortar placed over it, so the whole repair moves as one and shares stress instead of shearing along a hard plastic layer.
The product is designed as the priming step inside the wider Fosroc Renderoc concrete-repair system, not as a stand-alone finish. Applied correctly, it does two jobs at once: it improves adhesion of the repair mortar to the substrate, and it seals the thirsty, freshly-cut concrete so the substrate cannot pull mixing water out of the mortar and starve it of the moisture it needs to hydrate. That second effect matters enormously in Indian conditions, where a hot, dry, exposed surface will otherwise flash-dry the back of a repair and leave a weak, dusty bond. Renderoc BS is a small line item on any repair job, but skipping it is one of the most common reasons patch repairs debond within a season.
The technical insight
The bond in a cementitious repair is not glue — it is continued hydration across the joint. When a repair mortar is placed onto bare, absorbent concrete, the dry substrate acts like a sponge and sucks mixing water out of the mortar at the contact face. That water-starved skin never fully hydrates, so it cures soft and powdery and becomes the plane the repair later peels along. Renderoc BS interrupts that failure mechanism. The polymer-modified slurry is scrubbed into a saturated-surface-dry substrate, filling surface pores and micro-cracks and laying down a cement-rich film whose own paste is already gauged with the right amount of water. The polymer fraction improves wetting and adds a degree of flexural and adhesive toughness at the bond line. Crucially, the repair mortar must go on while this coat is still wet and tacky: at that stage the two cementitious pastes are continuous and hydrate into one monolithic matrix. If the slurry is allowed to dry to a hard skin before the mortar is placed, it stops being a bonding aid and becomes a bond-breaker — a set film the mortar simply sits on top of. Wet-on-wet timing is therefore not a nicety; it is the whole point of the product.
Where it is used
- Priming prepared concrete substrates immediately before Renderoc hand-applied patch repairs
- Bond coat over exposed, cleaned reinforcement steel within a repair zone
- Adhesion layer for vertical and overhead (soffit) repairs where slump and debonding are risks
- Bonding primer for reprofiling and fairing coats on spalled or carbonated concrete
- Interface treatment on bridge decks, piers and abutments during rehabilitation
- Priming inside water-retaining and water-treatment structures ahead of cementitious repair
- Bond coat on industrial floor and plinth repairs before mortar reinstatement
- Priming feathered edges and shallow patches that are especially prone to drying out
Key advantages
- Creates a chemically compatible cement-to-cement bond that moves with the concrete, unlike a rigid epoxy film
- Seals thirsty substrates so the fresh repair mortar keeps its mixing water and hydrates fully
- Single-component powder — only clean water (or Renderoc liquid) needed on site, simple to gauge
- Brush-applied and scrubbed in, so it wets out rough, porous and mechanically-prepared surfaces
- Formulated as an integral part of the Fosroc Renderoc system for guaranteed mortar compatibility
- Suits vertical and overhead work where a reliable bond line is hardest to achieve
- Improves the substrate/repair interface bond strength that governs long-term durability
- Low material cost relative to the repair it protects — cheap insurance against debonding
Limitations & considerations
- Strictly a wet-on-wet system — if the coat dries before the mortar is placed it becomes a bond-breaker and must be reapplied
- Not a repair mortar, waterproofing membrane or structural material in its own right; it only primes
- Bond performance depends entirely on substrate preparation — loose, laitance-covered or contaminated concrete will still fail
- Working and open time shorten sharply in hot weather, so batch sizes must match the pace of laying mortar
- Requires a saturated-surface-dry substrate — neither dusty-dry nor with free-standing water
- Cementitious chemistry means it should not be applied in the cold when temperatures are low and falling
Selection & best practice
Prepare first: mechanically break out unsound concrete to a square edge, expose and clean any corroded reinforcement, remove all dust and laitance, then pre-soak the substrate and bring it back to a saturated-surface-dry (damp, matt, no free water) condition. Mix only as much Renderoc BS as your team can place mortar over within its open time — that window is short and shrinks further on a hot, exposed surface, so batch small and batch often. Scrub the slurry firmly into the prepared surface with a stiff brush so it is worked into every pore and around exposed steel, at roughly 1.0-1.5 kg/m2 per coat rather than laid on as a thick film. Then place the Renderoc repair mortar immediately, while the bond coat is still wet and tacky. The single most common site error is letting the slurry skin over before the mortar goes on — if any area has dried, do not proceed; re-scrub a fresh coat onto that zone first. Follow the specific Fosroc technical data sheet for the exact Renderoc mortar being used, and cure the completed repair to protect it from rapid drying.
Specifications
| Type | Polymer-modified cementitious bonding slurry |
| Form | Single-component dry powder |
| Gauging | Clean water, or a Renderoc acrylic bonding liquid where specified |
| Mixed consistency | Creamy, brush-applicable slurry |
| Application rate | Approx. 1.0-1.5 kg/m2 per coat |
| Application method | Scrubbed in by stiff brush; repair mortar placed wet-on-wet |
| Compatible mortars | Fosroc Renderoc S, DS, HB, FC, LA and related repair mortars |
| Substrate condition | Saturated-surface-dry (damp, no free water) |
Industry use cases
Bridges & Highway Structures
Priming piers, girders, crash barriers and deck soffits before Renderoc patch repairs so reinstated concrete bonds monolithically and resists debonding under traffic vibration and thermal cycling.
Water & Wastewater
Bond coat inside tanks, sumps, channels and treatment structures where cement-based compatibility and a fully hydrated, watertight repair interface are essential.
Industrial & Commercial Buildings
Priming spalled columns, beams, plinths and soffits during structural repair and rehabilitation, particularly for vertical and overhead reinstatement work.
Marine & Coastal Assets
Interface treatment on chloride-attacked, carbonated concrete where a reliable bond around cleaned reinforcement is critical to the durability of the repair.
Repair & Maintenance Contractors
A low-cost, simple-to-mix priming step that dependably raises bond strength on routine patch repairs, protecting the value of the mortar and labour above it.
Frequently asked questions
Is Renderoc BS the same as Fosroc Renderoc Primer?
Can I skip the bonding slurry and apply repair mortar straight onto the concrete?
What happens if the slurry dries before I place the mortar?
How much Renderoc BS do I need per square metre?
Do I need to wet the concrete before applying it?
Does Space Arc supply the matching Renderoc repair mortar as well?
How we position Fosroc Renderoc BS
Renderoc BS is a priming step, not a finished repair, so it is almost always bought alongside the Renderoc repair mortar it primes — Fosroc Renderoc HB for high-build vertical and overhead reinstatement, Renderoc S/DS for hand-applied patching, or Renderoc FC as a fairing coat — plus a Nitobond bonding agent and a Fosroc curing membrane such as Concure to protect the finished work. Where a specifier prefers a factory-defined, standards-referenced bonding slurry over a site-gauged powder, the two-part Fosroc Renderoc Primer is the direct alternative. As a multi-brand distributor, Space Arc can also match this against equivalent bonding coats from other repair systems (for example Sika or MC-Bauchemie) where a project is standardised on another manufacturer — but for a Renderoc mortar repair, staying within the Fosroc family is the compatible choice.
Related Concrete Repair products
Source Fosroc Renderoc BS for your project
Authorised distribution, technical advice & professional application across Delhi NCR and Pan-India.