STP ShaliPoxy CTE 303 MI
Two-component moisture-insensitive coal tar epoxy that cures on damp concrete and moist steel — 300 micron DFT in a single coat, even above 85% RH.
What is it?
A two-pack, moisture-insensitive coal tar epoxy coating from STP Limited that builds 300 ± 30 micron DFT per coat over damp (green) concrete and moist metal surfaces.
Where is it used?
Sewage treatment plants, MS and concrete pipelines, below-grade structures and any concrete or steel surface that cannot be dried before coating.
Why use it?
It bonds and cures where standard epoxies blister — substrates above 85% RH with moisture up to 15% — backed by 2,000-hour salt spray endurance and 3.8 MPa pull-off adhesion on concrete.
How is it applied?
Machine-mix Components A and B at 3:1 by volume, then brush, roller or airless-spray at 350–400 micron wet film; recoat within the 8–12 hour overcoat window if a second coat is specified.
About STP ShaliPoxy CTE 303 MI
Site programmes in India rarely give concrete the luxury of drying out. Tank walls get stripped and handed over mid-monsoon, below-grade structures stay permanently damp, and steel in coastal plants carries condensation for most of the year. ShaliPoxy CTE 303 MI from STP Limited is formulated for exactly these conditions: a two-component coal tar epoxy built on a moisture-insensitive hardener system that wets out, bonds and cures on damp green concrete and moist mild steel, and tolerates ambient humidity above 85 percent.
The performance is tested, not just claimed. The cured film records 3.8 MPa pull-off adhesion on concrete — the substrate fails before the coating does — and 6.0 MPa on moist metal, survives 2,000 hours of salt spray at 300 micron DFT, and comes through 30-day immersion in demineralised water, 1% sulphuric acid and 1% caustic soda without blistering under the AWWA C 210 protocol. Space Arc Engineering stocks ShaliPoxy CTE 303 MI in 20 L packs across Delhi NCR and Pan-India, with trained applicator crews available for project execution.
The technical insight
A conventional epoxy fails on damp substrates because water competes with the resin at the bond line: the hardener reacts with surface moisture instead of the epoxy, causing blushing, poor crosslinking and eventual disbondment as vapour pressure lifts the film. CTE 303 MI counters this on two fronts. Its special hardener keeps curing with the epoxy resin even in the presence of free water, so the film cures through the dampness rather than fighting it. Meanwhile the coal tar fraction extends the cured network with an intensely hydrophobic binder — water absorption stays below 0.6 percent and coating resistance holds around 10⁹ ohm·cm², starving corrosion cells of the electrolyte path they need. At 75 percent volume solids, a single 350–400 micron wet pass reliably leaves 300 microns dry.
Where it is used
- Protective coating over damp or green concrete where the programme cannot wait for substrate drying
- Concrete and steel surfaces in sewage treatment plants — channels, tank exteriors, screens and pipework
- External corrosion protection of mild steel and concrete pipelines
- Steel structures and equipment operating above 85% RH, including coastal and humid process plants
- Below-grade structures — basements, culverts, retaining walls — carrying permanent residual moisture
- Refurbishment of corroded steelwork where full surface drying cannot be guaranteed
- Immersed non-potable duty with dilute acid and alkali exposure, validated by 30-day immersion testing
Key advantages
- Cures on damp concrete and moist steel — no forced drying or moisture-meter hold points
- High build: 300 ± 30 micron DFT in a single coat at 75 ± 3% volume solids
- Adhesion beyond the substrate: 3.8 MPa pull-off on concrete (cohesive concrete failure) and 6.0 ± 0.5 MPa on moist metal
- 2,000-hour salt spray pass and ISO 20340 cyclic-corrosion pass at 300 micron DFT
- No blistering, peeling or disbonding after 30-day immersion in DIW, 1% H2SO4 and 1% NaOH
- Flexible film passes the 1/8-inch mandrel bend and 45 cm impact test, tolerating thermal shock and site knocks
- Resists micro-organism growth (ASTM G 21) — a real advantage in sewage and effluent service
- Applies by brush, roller or airless spray with a workable 3–4 hour pot life
Limitations & considerations
- Contains coal tar and is supplied in black only — unsuitable where a decorative or light-coloured finish is needed, and strict PPE discipline is essential
- Not recommended for surfaces in contact with potable water or foodstuffs
- Chalks on prolonged sunlight exposure; STP confirms it is cosmetic, but the grey cast shows on exposed structures
- Damp is fine, wet is not — rain, ponding or standing water during and after application causes incomplete cure
- Application window is 10–40°C substrate and ambient; below about 12°C mixed-material temperature, viscosity climbs sharply
- Flammable in the wet state (flash point 30–38°C) — ventilation and ignition control are mandatory in confined spaces
Selection & best practice
Wire-brush off dust, laitance, unsound plaster, oil, grease, corrosion deposits and algae; roughen steel by sand blasting, then clean by vacuum or water washing and let excess water drain — the surface may be damp but must never carry standing water. Stir each component separately, combine A and B at 3:1 by volume, and machine-mix for 3–5 minutes with a slow-speed stirrer keeping the paddle submerged; never mix by hand and never add thinner, which kills sag resistance and DFT. Apply by brush, roller or airless spray (60:1 pump ratio, 23-thou tip, 3,000–4,000 psi) at 350–400 micron wet film. Recoat after 5–8 hours at 30°C; if the second coat slips past 16 hours, solvent-wipe the surface first. Clean tools immediately with STP Thinner.
Specifications
| Mixing ratio (Comp A : Comp B) | 3 : 1 by volume |
| Theoretical coverage | 2.5 m²/L at 300 ± 30 micron DFT |
| Recommended wet film thickness | 350–400 micron per coat (ASTM D 4414) |
| Volume solids | 75 ± 3% (ASTM D 2697) |
| Pot life at 30°C | 3–4 hours (ASTM D 2471) |
| Touch dry / overcoat window | 2–3 hours / 8–12 hours at 30°C |
| Full cure | 7 days |
| Adhesion, pull-off (ASTM D 4541) | 3.8 MPa on concrete; 6.0 ± 0.5 MPa on moist metal |
| Pack size / shelf life | 20 L (15 L Comp A + 5 L Comp B); 12 months at 5–38°C |
Industry use cases
Water & Wastewater
STP and ETP structures stay damp for life. CTE 303 MI coats clarifier walls, channels and steel screens without a drying hold, and ASTM G 21 resistance keeps microbial growth off the film.
Pipelines & Utilities
External protection for MS and concrete pipe runs, valve chambers and pumping-station steelwork, with AWWA C 210-protocol immersion performance behind the specification.
Coastal & Marine Industry
Plants in the salt-laden coastal belt sit above 85% RH for months at a stretch; the 2,000-hour salt-spray and cyclic-corrosion results carry the specification for shore-side steel and concrete.
Below-Grade Infrastructure
Culverts, subways, lift pits and retaining structures hold residual moisture that wrecks standard epoxies — here the coating goes straight onto the damp face.
Industrial Maintenance
Shutdown windows are short. Maintenance teams coat freshly washed, still-moist steel and concrete immediately after cleaning instead of losing days waiting for it to dry.
Frequently asked questions
How damp can the concrete actually be?
Can I use it inside a potable water tank?
How many coats do I need?
What if recoating gets delayed?
Will the black finish stay black outdoors?
What does a 20 L pack cover?
How we position STP ShaliPoxy CTE 303 MI
Within the STP range, CTE 303 MI is the grade you specify when the substrate is damp: the standard ShaliPoxy CTE 303 delivers the same 300 micron single-coat coal tar epoxy build at lower cost once concrete or steel is dry, and sister CTE grades (103, 104, 410, 503) cover other film builds and pipeline coating duties. For economical anti-corrosive protection where a high-build epoxy is more than the job needs, STP’s coal-tar-based ShaliMastic HD is the budget option. From our Fosroc line, Nitocote ET402 is the 100% solids coal tar epoxy typically specified for immersed seawater and effluent structures. Space Arc supplies all of these ex-stock and can match the grade to your exposure conditions.
Fosroc Nitocote ET402 — Specifiers pick Nitocote ET402 when they want a solvent-free, 100% solids coal tar epoxy for immersed seawater or effluent duty and can present a properly dried substrate — CTE 303 MI wins whenever the concrete or steel cannot be dried first.
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