Liquid epoxy coatings — plural component spray, brush-applied novolac, and 100 percent solids hand-mix products — are the workhorses of pipeline rehabilitation, HDD coating, and field repair. They are also the most application-sensitive coatings in common use on pipelines today. The same product can deliver 30 years of service in one installation and fail in 18 months in another. The variable is almost always how it was applied.
This article walks through the failure modes we see most often when investigating liquid epoxy disbondment, blistering, and underfilm corrosion on buried pipelines, with the field controls that prevent each one.
Why Liquid Epoxies Get Used in the First Place
Liquid epoxies fill the gaps that plant-applied systems leave open. They are used for:
- Rehabilitation of legacy coatings (coal tar, asphalt enamel, tape) where the line cannot be replaced.
- HDD coatings where the pull stress and abrasion exceed what FBE alone can survive.
- Field joint coating on pipelines where plant coating is FBE, 3LPE, or 3LPP.
- Repair of mainline coating damage after transport or backfill.
- Tank, vessel, and fitting coating where geometry rules out plant application.
Each of these use cases stresses the coating differently. A failure mode that is rare in one application is dominant in another.
Failure Mode 1: Mix Ratio Out of Specification
Liquid epoxies are two-part products. The ratio between resin and hardener is set by chemistry, not by convenience. A product specified at 4:1 by volume that is applied at 5:1 will cure to a film that looks correct but will never reach its specified glass transition temperature, chemical resistance, or adhesion.
- Plural component spray: Mix ratio is set by the pump heads. Ratio checks must be performed at the start of every shift and after every product change, with the gun firing into separate containers and the two streams weighed.
- Hand-mix: Whole-kit mixing is mandatory. Splitting a kit by eye is the single most common cause of soft, never-fully-cured patches on field joints and repairs.
- Field evidence: Soft or rubbery films, thumbnail-deformable patches, and disbondment that does not show the clean steel of an adhesion failure but rather a coating that pulls away in sticky sheets.
Failure Mode 2: Application Outside the Temperature Window
Every liquid epoxy has a specified substrate temperature range. Spray below the minimum and the coating cures too slowly, traps solvent, and fails to crosslink. Spray above the maximum and the pot life collapses, the gel time at the gun tip falls below what the equipment can handle, and the film cures with internal stress that drives cracking under thermal cycling.
The most-missed control is dew point. Application onto steel within 3 degrees C of the dew point allows a microscopic moisture film between the steel and the coating. The product cures around it. The bond is to water, not to steel.
Failure Mode 3: Recoat Window Violations
Most liquid epoxies require a second coat or topcoat applied within a defined window — typically 8 to 24 hours, depending on temperature. Apply too soon and solvent is trapped between coats. Apply too late and the surface has cured past the point where the next coat can chemically bond, leaving an intercoat adhesion that is purely mechanical.
- Field evidence: Clean delamination between two coats of the same product, often visible during ILI dig.
- Root cause: Crew scheduling that did not respect the recoat window, or unmeasured ambient temperature changes that shifted the window forward.
- Control: Log the time of first coat, the temperature trend, and the recoat window for each section. If the window is missed, the surface must be abraded and tested for adhesion before topcoat.
Failure Mode 4: Dry Film Thickness Outside Specification
Liquid epoxies fail at both ends of the DFT range. Below the minimum specified film build, the coating lacks the barrier thickness to resist water vapor transmission and CP shielding. Above the maximum, the coating cures with internal stress, develops mud-cracking on convex surfaces, and frequently disbonds at the edges.
The most common DFT errors we see are:
- Wet film thickness checks skipped, with the applicator targeting visual coverage instead of measured build.
- DFT gauge not zeroed against the actual blasted profile of the substrate.
- Plural component pump pressure drift between calibration checks, causing the actual film build to drift away from the target.
Failure Mode 5: Inadequate Surface Preparation
Liquid epoxies require an anchor pattern in the range of 2 to 4 mils, near-white metal blast cleanliness (SSPC-SP10 / NACE No. 2), and a chloride level below the contractual limit. The surface tolerance of modern surface-tolerant epoxies is often oversold by manufacturers and misread by applicators. Surface tolerant does not mean surface preparation optional.
In rehabilitation work where mechanical preparation (needle gun, power tool to SSPC-SP11) is the only option, the inspector must verify that the limits of the product's surface preparation specification are honored — not the limits of what the crew was equipped to do.
Failure Mode 6: Holidays at Geometry Changes
Field joints, fitting transitions, and welds present geometry that liquid epoxy is prone to under-coating. The cusp at the toe of a weld, the edge of a flange, and the shoulder of a bend all develop films that are 30 to 50 percent thinner than the specified DFT unless the applicator is deliberately stripe-coating these features.
Holiday detection per NACE SP0188 at 100 percent of the coated surface, with the voltage set for the actual DFT, is the only reliable way to find these and repair them before backfill.
Pre-Qualification: The Step Most Projects Skip
For any project using liquid epoxy at scale, applicator pre-qualification on a test spool — under the same temperature, humidity, and equipment conditions as the production work — is the single highest-value inspection investment. The pre-qualification spool reveals every failure mode above before it is buried in the trench.
Related reading: FBE coating failure modes covers the plant-applied counterpart, and preventing HDD coating damage during pullback covers the mechanical stress that liquid epoxy ARO systems must survive.
How Raphoon Helps
We provide application inspection, pre-qualification supervision, and post-failure investigation for liquid epoxy systems used in pipeline rehabilitation, HDD, and field coating. Our inspection staff have run plural component equipment in the field — the inspection criteria translate to actions the crew can take, not theoretical objections. See our inspection services or contact us for project-level support.