A protective coating on a structure is inspected. A lining inside a tank is tested, electrically, across every square metre of its surface. That difference decides how the work is scoped, who is in the vessel, and what happens when the result comes back positive.
Why a Lining Is a Different Product From a Coating
An external coating system on structural steel works statistically. Small imperfections are tolerable because the environment is intermittent, the steel has reserve thickness, and the next maintenance cycle will find what the last one missed. A lining in immersion service does not work that way. One pinhole through to the substrate in a tank holding an electrolyte is a complete corrosion cell: anode, cathode and conducting path, concentrated at a single point.
Because the defect is concentrated, the attack is concentrated. A square metre of sound lining does not share the load. The pinhole corrodes locally, undercuts the surrounding film, and the failure spreads from a point that was invisible on the day the work was signed off.
A pinhole in an external coating is a blemish. The same pinhole in an immersion lining is a corrosion cell with the whole tank contents behind it.
That is why linings are specified with holiday detection across the finished surface rather than with spot inspection, and why the test rather than the application is the acceptance event.
The Work Inside the Tank Is Confined Space Work
Before any of the testing matters, there is the question of who is inside the vessel and under what controls. Lining a tank is confined space work, and the coating scope has to absorb that rather than treat it as somebody else’s provision.
Three things make it more demanding than ordinary entry. Solvent vapour from the coating is being generated continuously during application, so the atmosphere is being actively degraded by the work itself rather than merely being unventilated. Many solvent vapours are heavier than air and accumulate in the lowest part of the vessel, which is exactly where an applicator working a tank bottom is positioned. And the vapour is both a toxicity problem and a flammable atmosphere problem, so monitoring has to cover oxygen, flammable gas and specific contaminants rather than oxygen alone.
Forced ventilation sized for the work. Air movement calculated against the solvent being released during application and cure, not general ventilation sized for an empty vessel.
Continuous atmospheric monitoring. Oxygen, flammable gas and relevant contaminants, monitored throughout rather than tested once at entry.
Rescue cover in place before entry. Standby personnel and retrieval arrangements that can extract someone from the actual geometry of that vessel, through that manway.
Ignition control. Equipment, lighting and static management appropriate to a flammable atmosphere, including during the cure period when the vessel is unattended.
Cure period controls. The vessel does not become safe when the applicators leave. Solvent continues to evolve, and entry controls apply until monitoring says otherwise.
The last item causes more incidents than the application phase does, because the hazard is invisible and the work appears finished. A vessel that has been left overnight with the ventilation switched off can hold a flammable atmosphere in the morning.
Holiday Detection, and the Rule That Is Not in Any Standard
Holiday detection locates discontinuities by looking for an electrical path through the film to the conductive substrate. There are two methods, and the thickness of the lining decides which one applies.
ASTM D5162, Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates, defines Method A, low voltage wet sponge testing, and Method B, high voltage spark testing. NACE, now AMPP, SP0188, Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates, covers the same ground with the voltage selection procedure. ASTM G62 covers pipeline coatings specifically.
The dividing line is 20 mils, 500 micrometres. At or below that, low voltage wet sponge testing applies, using a prescribed low test voltage and a wetted sponge drawn across the surface. Above it, high voltage spark testing applies, with the voltage calculated from the measured film thickness. High voltage methods can be used on thinner films, down to around 10 mils or 250 micrometres, provided the voltage is set correctly for the thickness.
The most widely used number in holiday detection appears in none of the standards that govern it, and testing to it has been shown to leave holidays undetected.
Here is the part worth knowing. Generations of inspectors have been taught the rule of thumb of 100 volts per mil of coating thickness. It is not in the AMPP standard and it is not in the ASTM standard. It never was. Research published in 2021 and 2022 demonstrated that testing at that level will likely leave holidays undetected, because the voltage is insufficient to break down the air gap at a genuine discontinuity under field conditions.
The standards were revised in response. ASTM G62 incorporated an updated voltage method in its 2023 revision, followed by AMPP/NACE SP0188 and ASTM D5162 in their 2024 revisions. The revised documents set out the calculation and also provide lookup tables for common thicknesses in both mils and micrometres, so the figure does not have to be derived on site.
The practical consequences for a specification are direct. State the standard and the revision rather than naming a voltage, since the voltages moved. Require the test voltage to be calculated from measured film thickness rather than assumed. And confirm the coating manufacturer’s maximum permitted test voltage for that product, because the governing limit is whichever is lower.
The concern that higher voltages will burn through sound lining is largely misplaced. Modern linings subject to holiday testing typically have dielectric strength well above 500 volts per mil, while the revised calculation for a 20 mil film works out considerably below that. The margin exists. What the old rule of thumb lacked was margin in the other direction, toward detection.
What a Positive Result Actually Means
A detected holiday is a repair, and the repair has to be done to the same standard as the original work, which is the step that gets compressed when the vessel is wanted back in service.
Cure Is Not the Same as Dry
The final step is the one most often shortened under schedule pressure, and it is where immersion service differs sharply from atmospheric exposure.
A lining that is touch dry has not developed its chemical resistance. Full cure for a high build immersion lining depends on temperature, film thickness and ventilation, and a vessel returned to service before it is achieved will show softening, blistering or premature breakdown in a way that looks like a product failure and is not. Where elevated temperature cure is specified, it is specified for a reason and skipping it changes the product’s performance.
For a tank in a tropical climate, ambient temperature works in favour of cure times while high humidity works against application conditions, so the two have to be managed together rather than assumed to cancel out. The same substrate temperature and dew point records that govern application also tell you whether the cure conditions were met.
None of this is exotic, and all of it is cheaper specified than argued. A lining scope that names the holiday detection standard and revision, requires calculated test voltage, builds confined space provision into the coating work rather than alongside it, and holds the vessel until cure is verified, is a scope where the acceptance test means something. Our industrial painting and coating service is built around that sequence, and the linings supplied under it carry type approvals appropriate to the service they are specified for.
Frequently Asked Questions
What is holiday detection and why is it used on linings?
It is an electrical test that locates discontinuities by detecting a path through the film to the conductive substrate. It is specified for linings in tanks, vessels and piping because a pinhole in immersion service creates a complete corrosion cell at a single point, where a comparable defect in an external coating would be tolerable. ASTM D5162 and AMPP/NACE SP0188 are the governing practices.
When is wet sponge used rather than spark testing?
Film thickness decides it. At or below 20 mils, 500 micrometres, low voltage wet sponge testing applies, which is Method A in ASTM D5162. Above that, high voltage spark testing applies, Method B, with the voltage calculated from measured thickness. High voltage methods can be used on thinner films, down to around 250 micrometres, provided the voltage is set correctly.
Is the 100 volts per mil rule correct?
No, and it does not appear in either the AMPP or the ASTM standards despite being widely taught. Research published in 2021 and 2022 showed that testing at that level will likely leave holidays undetected. The standards were revised in response, with ASTM G62 in 2023 and AMPP/NACE SP0188 and ASTM D5162 in 2024, and the revised documents include voltage lookup tables alongside the calculation.
Why is tank lining treated as confined space work?
Because the work itself degrades the atmosphere. Solvent vapour is generated continuously during application, many such vapours are heavier than air and collect where the applicator is working, and the result is both a toxicity and a flammable atmosphere hazard. Forced ventilation, continuous monitoring, rescue cover and ignition control belong inside the coating scope, and the controls continue through the cure period after the applicators have left.
Sources: ASTM D5162, Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates, Method A low voltage wet sponge and Method B high voltage spark, 2024 revision · AMPP (formerly NACE) SP0188, Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates, 2024 revision, including the test voltage selection procedure · ASTM G62, Standard Test Methods for Holiday Detection of Coatings used to Protect Pipelines, 2023 revision, which first incorporated the updated voltage method · Published research of 2021 and 2022 establishing that testing at 100 volts per mil will likely leave holidays undetected, which prompted those revisions
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