STP ShaliPoxy CTE 503
Two-component polyamide-cured coal tar epoxy that builds 500 microns in a single coat — flexible, abrasion-resistant protection for buried and immersed steel and concrete.
What is it?
A 2K polyamide-cured, cold-curing coal tar epoxy protective coating from STP Limited, applied direct to metal without a primer and reaching 500 micron DFT in one wet-on-wet coat.
Where is it used?
Buried pipelines, crude oil storage tanks, sheet and pipe piling, penstocks and barrage gates, sewage treatment structures, foundation walls, sumps, and chemical plant steelwork.
Why use it?
Combines the water impermeability of coal tar with the flexibility of a polyamide cure — 15-20% elongation, over 5.5 MPa adhesion, and compliance with AWWA C-210:2015 and SSPC Paint 16.
How is it applied?
Mix components 3:1 by volume, then apply by brush, roller or airless spray onto blast-cleaned steel or cured concrete. One coat gives 500 microns; recoat within 8-30 hours at 30°C.
About STP ShaliPoxy CTE 503
ShaliPoxy CTE 503 is the flexible, high-build grade in STP Limited’s coal tar epoxy range. It is a two-component system — a coal tar epoxy base cured with a polyamide converter — engineered for steel and concrete that spends its service life buried, submerged, or under constant chemical and abrasive attack. A single wet-on-wet pass delivers 500 microns of dry film, and on properly blast-cleaned steel it goes on direct to metal with no primer, which cuts a full stage out of pipeline and tank coating programmes.
At 95 ± 2% volume solids, very little of what you spray evaporates away; the applied film is nearly all barrier. The coating conforms to AWWA C-210:2015, SSPC Paint 16 and Corps of Engineers C200/C200a, so it slots directly into water-infrastructure and oil-and-gas specifications that call up these standards. Space Arc Engineering, as authorised distributor and applicator, supplies 20 L and 200 L drums with application support across Delhi NCR and pan-India project sites.
The technical insight
Coal tar pitch and epoxy resin do complementary jobs in this film. The epoxy provides the crosslinked backbone and adhesion; the tar fraction fills the cured matrix with a hydrophobic, plate-like phase that gives coal tar epoxies their famously low moisture-vapour permeability. What makes CTE 503 different from a standard amine-cured CTE is the polyamide hardener: its longer, amide-linked chain segments build a tougher, more elastic network — 15-20% elongation and good ASTM D522 flexibility instead of a hard, brittle glass. That elasticity is why the coating shrugs off thermal shock and impact, records only 112 mg loss in Taber abrasion testing, and holds cathodic disbondment under 10 mm with dielectric strength above 250 V/mil — the properties that matter on cathodically protected buried pipelines.
Where it is used
- External coating of buried steel pipelines and underground structures
- Crude oil storage tank protection in terminals and refineries
- Sheet piling and pipe piling in marine, riverine and reclaimed ground
- Dam gates, barrage gates and penstocks in continuous water contact
- Concrete and steel surfaces in sewage and effluent treatment plants
- Foundation walls, basements and sumps exposed to ground moisture
- Structural steel in paper mills and chemical process plants
- Abrasion-resistant lining where sediment or slurry wears standard coatings
Key advantages
- 500 micron DFT in a single wet-on-wet coat — fewer passes, faster programmes
- Direct-to-metal on blast-cleaned steel; no separate primer stage required
- Complies with AWWA C-210:2015, SSPC Paint 16 and Corps of Engineers C200/C200a
- 95 ± 2% volume solids, so applied film thickness barely shrinks on cure
- Polyamide cure gives 15-20% elongation — tolerates movement, impact and thermal shock
- Passes 1000-hour salt spray and holds cathodic disbondment below 10 mm
- Proven in 30-day immersion in water, 1% sulphuric acid and 1% caustic without blistering
- Handles service temperatures up to 95°C continuous and brief excursions to 120°C
Limitations & considerations
- Available in black semi-gloss only; like all coal tar epoxies it chalks and discolours under sustained UV, so it is not a cosmetic or colour-stable finish
- Coal tar chemistry demands strict PPE and ventilation, and some client specifications now restrict coal tar products — confirm acceptability before ordering
- Not certified for potable water contact; for drinking-water immersion duty verify suitability with STP or specify an alternative system
- Application window is 10-49°C: at 10°C full cure stretches to 15 days, so cold-weather programmes need schedule allowance
- Pot life is 4 hours on a 100 g sample; large mixed kits in Indian summer heat will stiffen sooner, so batch accordingly
- Maximum recoat interval is 30 hours at 30°C — a missed window means roughening the first coat before overcoating
Selection & best practice
Preparation decides how long this coating lasts. Blast-clean or mechanically grind steel and remove every trace of rust, oil and dust; new concrete must be at least 28 days old and dry. Power-stir each drum separately, then combine the full pre-weighed A and B components at 3:1 by volume with a slow-speed stirrer for five minutes, keeping the blade submerged to avoid dragging in air. Let the mix stand five minutes to release entrapped air before application. Brush and roller work for touch-up and small areas; production work goes fastest with airless spray using a 0.66-0.98 mm tip at over 6000 psi with zero dilution. If your pump needs thinning, cap it at 10% STP Thinner and remember diluted material builds less DFT per pass. At 30°C expect touch-dry in 4 hours, a recoat window of 8-30 hours, and full cure in 7 days. Clean tools immediately with STP Thinner.
Specifications
| Mixing ratio (A:B, by volume) | 3 : 1 |
| Volume solids | 95 ± 2% |
| Theoretical coverage | 2 m²/L per coat at 500 micron DFT |
| Pot life (100 g sample, ASTM D2471) | 4 hours |
| Touch dry / recoat at 30°C | 4 hours / 8-30 hours |
| Elongation (ASTM D638) | 15-20% |
| Adhesion, pull-off (ASTM D4541) | > 5.5 MPa |
| Standards compliance | AWWA C-210:2015, SSPC Paint 16, CoE C200/C200a |
| Packing / shelf life | 20 L and 200 L drums; 12 months at 5-38°C |
Industry use cases
Oil & Gas
External protection of buried cross-country pipeline sections and crude oil tank farms, where cathodic disbondment below 10 mm and dielectric strength above 250 V/mil keep the coating compatible with CP systems.
Water Infrastructure & Dams
Penstocks, barrage gates and dam steelwork in permanent water contact — the polyamide film flexes with gate operation instead of cracking like harder amine-cured systems.
Sewage & Effluent Treatment
Concrete tanks, channels and submerged steel in STPs and ETPs, where 30-day immersion resistance to dilute acid and alkali handles aggressive effluent chemistry.
Marine & Ports
Sheet piling, pipe piling and jetty steel in splash and immersion zones — 1000-hour salt spray performance and strong abrasion resistance against fender and debris contact.
Industrial Plants
Structural steel, sumps and foundations in paper mills and chemical plants exposed to spillage, washdown and process fumes.
Frequently asked questions
How much area will a 20 L drum cover?
Do I need a primer under CTE 503?
What is the difference between CTE 503 and a standard coal tar epoxy like CTE 103?
How soon can I apply the second coat?
Can it be applied by airless spray, and can I thin it?
Is it suitable for permanently submerged service?
How we position STP ShaliPoxy CTE 503
Within the STP family, ShaliPoxy CTE 503 is the flexible immersion-grade choice: pair it with ShaliPrime Zn Ph 60 where a zinc phosphate primer is specified, keep STP Thinner on site for spray adjustment and tool cleaning, and step across to standard ShaliPoxy CTE grades where a harder amine-cured film suits static, non-flexing structures. For projects whose consultants specify Fosroc, Nitocote ET140 from our Fosroc range covers the same buried and immersed steel-and-concrete duty. Space Arc supplies both brands with applicator support across Delhi NCR and pan-India.
Fosroc Nitocote ET140 — Specifiers standardised on Fosroc systems can achieve the same coal tar epoxy protection for immersed and buried steel or concrete with Nitocote ET140.
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