Coating India's Metro Expansion: Specification Lessons from the Field

India is building metro rail at a pace few countries have attempted. Dozens of cities are at some stage of construction, extension or planning, and the pipeline extends well past 2030. For coating manufacturers and applicators this is one of the largest sustained sources of demand in the country.

It is also technically awkward work, because a metro project is not one coating job. It is at least five, executed on the same site under one contract.

Five distinct coating problems in one project

Elevated viaduct structural steel. Long-span steel over roads and waterways, in urban atmospheres with vehicle exhaust and often within reach of coastal air. High durability is demanded because access for maintenance is severely constrained — you cannot close a running viaduct to repaint it easily. This pushes specifications toward C4 or C5 systems with 20 to 25 year durability.

Concrete piers, segments and stations. Reinforced concrete, where the dominant deterioration mechanism is not general corrosion but carbonation and chloride ingress reaching the reinforcement. The correct product family is anti-carbonation coatings assessed for CO₂ diffusion resistance, not conventional exterior emulsion.

Underground stations and tunnels. Permanently damp, poorly ventilated, with restricted working windows and severe fire and smoke requirements for materials in enclosed public spaces. Low-VOC and often solvent-free systems are mandatory here for occupational reasons alone.

Rolling stock. Coach exteriors need long-term gloss and colour retention against UV and repeated cleaning, and interiors must satisfy fire, smoke and toxicity requirements — typically EN 45545 hazard levels for enclosed passenger transport.

Depot and workshop flooring. Heavy point loads, hydraulic fluid and oil exposure, and maintenance windows that are measured in hours rather than days.

Treating these as a single package, with one product range applied to all of it, is where specifications go wrong.

The interfaces are where failures start

The steel-to-concrete interface at bearings and pedestals is a classic problem area. Water collects, dries slowly, and the two substrates need different systems that must nonetheless terminate cleanly against one another.

Bolted and welded connections on viaduct steel are the second. The specification says stripe coat; the site often does not, because it is slow, manual and invisible once the full coat goes on. Nearly every premature viaduct coating failure begins at an edge, a bolt head or a weld.

The third is the boundary between contractors. Structural steel is coated in a fabrication shop under controlled conditions. It is then transported, handled, erected, and welded on site, damaging the coating at every stage. The site touch-up specification — how damage is prepared, what product is used, what film thickness is restored — is frequently underspecified and almost always underexecuted. A shop-applied system to C5 standard with touch-up done casually with a brush and whatever primer is available is no longer a C5 system.

Access is the economic argument

The reason metro specifications demand high durability is not that the atmosphere is unusually aggressive. It is that maintenance access is expensive and disruptive.

Repainting a viaduct over an operating road requires traffic management, access platforms, containment to prevent debris falling on the public, and night working. The direct coating cost is a small fraction of that exercise. When the cost of access dominates, the correct economic decision is to specify a system that defers the access for as long as possible.

This is the argument that persuades value-engineering discussions when a general appeal to quality does not: not that the better system is better, but that the cheaper system commits the operator to an expensive intervention years earlier.

Documentation is a deliverable

Metro authorities and their consultants run structured vendor registration and approval processes. Products are not accepted on reputation. What is needed, usually well before award:

Product data sheets with complete technical parameters

Independent test reports against the specified standards

Certificates of relevant approvals and registrations

Reference project lists with contactable clients

Manufacturing capacity and quality system certification

Batch test certificates issued against each supply

Manufacturers who assemble this in advance move quickly when a tender appears. Those who begin assembling it after the tender is floated do not participate in that tender.

What to specify, briefly

For elevated structural steel in an urban Indian environment: assess honestly between C4 and C5 based on distance from coast and local industry; specify high or very high durability given the access constraint; zinc-rich or inorganic zinc silicate primer; a micaceous iron oxide or high-build epoxy intermediate; and a polyurethane or polysiloxane finish. Specify the stripe coat explicitly, specify the site touch-up procedure explicitly, and specify who verifies dry film thickness and against what standard.

The last two sentences matter more than the product names.