Frequently Asked Questions

What are the limitations of Nitoseal PU compared to the two-component Thioflex 600 polysulfide sealant and when should the two-component system be specified?

Nitoseal PU single-component polyurethane and Thioflex 600 two-component polysulfide occupy different performance tiers in the Fosroc sealant range, and the difference between them has practical implications for long-term joint performance in critical applications. UV resistance and service life: Nitoseal PU is UV-sensitive — in direct sunlight exposure on an external building facade, the polyurethane surface oxidises progressively, causing surface chalking and cracking of the outer skin of the sealant; over time (typically 7 to 12 years in exposed Indian external conditions), the UV degradation of the PU sealant surface reduces the effective elongation of the surface zone, leading to joint failure by cracking through the sealant body in very UV-exposed locations. Thioflex 600 polysulfide has inherently superior UV resistance and typically achieves 20 to 30 years of service life in comparable exposure. Cure reliability: Nitoseal PU cures from atmospheric moisture and may cure slowly or unevenly in deep joints, closed joints, or in dry air conditions (below 30 per cent relative humidity — possible in Delhi NCR December-January). Thioflex 600 two-component cures from a chemical reaction regardless of air moisture content, providing reliable cure in all conditions. Water-retaining and chemical resistance: Nitoseal PU has limited suitability for permanent water immersion and chemical contact; Thioflex 600 has a potable water contact grade and a jet fuel contact grade providing specific service environment performance. For these reasons, Thioflex 600 is specified where: the joint is in a critical structure with a design life above 30 years; the joint is exposed to UV, permanent water, or chemical contact; and where the cure reliability of a two-component system is required. Nitoseal PU is appropriate for general building maintenance, shorter design life applications, and simple joint sealing where cost, convenience, and paintability are the primary selection criteria.

Does Nitoseal PU require a primer for application on smooth concrete and common Indian building materials like brick and ceramic tile?

The priming requirement for Nitoseal PU depends on the substrate type and its porosity and surface energy. For most common porous building materials encountered in Indian construction — ordinary wire-cut brick, concrete block, rough or lightly formed concrete, natural stone, and sand-faced plaster — Nitoseal PU has adequate adhesion without a primer because the porous surface provides both mechanical anchoring of the cured sealant and sufficient reactive surface chemistry for the PU adhesion mechanism. For smooth, dense, or non-porous substrates — polished granite or marble, smooth glass, anodised aluminium, galvanised steel, glazed ceramic or porcelain tiles, and very smooth formed concrete with no surface texture — a primer is required to provide the molecular surface preparation that allows the PU adhesion mechanism to work. On glass, aluminium, and most metals, a solvent-based silane or titanate primer (such as Fosroc Primer 19 or equivalent) cleans the surface and provides the coupling agent needed for PU adhesion. On very smooth concrete, a thin coat of solvent-based primer similarly prepares the surface. A simple field test for whether primer is needed: apply a small test bead of Nitoseal PU to the cleaned substrate, cure for 7 days, and peel the sealant back — if the failure is cohesive (tears within the sealant material), no primer is needed; if the failure is adhesive (peels cleanly at the sealant-to-substrate interface), a primer is required. This peel test should be done on any substrate type where primer requirement is uncertain before committing to a large sealing project.

How should old, failed Nitoseal PU or other polyurethane sealant be removed from a joint before resealing with fresh sealant and what preparation is needed?

Removing old failed sealant from a joint and preparing the joint for resealing is a necessary maintenance operation in building management — polyurethane sealants typically require replacement after 10 to 15 years in exposed external joints as UV degradation reduces their elongation and the sealant begins to crack or debond. The removal and preparation procedure is: old sealant removal — pull out the bulk of the old sealant from the joint by hand (grab the exposed sealant bead and pull — most failed sealant pulls out in sections); remove residual sealant adhering to the joint faces by carefully cutting with a sharp craft knife or oscillating multi-tool, making parallel cuts 2 to 3 mm from each joint face and removing the thin strip of old sealant between the cut and the face; be careful not to cut into the concrete or masonry joint face, as this increases the joint width and wastes sealant; old backing rod (if present) should be removed and replaced with a new closed-cell polyethylene backing rod sized to the joint width; clean the joint faces of all residual sealant, dust, and old primer using a solvent-dampened cloth (acetone or MEK) and allow to dry; inspect the joint faces for damage — if old sealant adhesion has pulled away mortar or concrete surface layer, the joint face damage must be made good with compatible repair mortar before resealing; install the new backing rod to the correct depth for the new sealant (joint depth approximately half the joint width); apply primer if required for the specific joint face substrate; apply fresh Nitoseal PU (or Thioflex 600 or Nitoseal MS600 depending on the performance upgrade specification) and tool to concave profile. The quality of old sealant removal determines the adhesion life of the new sealant — residual old sealant at the joint face prevents the new sealant bonding directly to the substrate, creating an immediate adhesion weakness.