Shop primer is the only coating on a ship chosen for how well it tolerates being destroyed. It has to survive months in a fabrication shop and then be welded and cut straight through, and the choice made at that stage decides how much blasting someone pays for years later.
A Coating With Two Incompatible Jobs
Steel arrives at a fabrication shop blasted and bare, and bare blasted steel begins rusting in hours in a Malaysian atmosphere. Shop primer exists to hold that surface through cutting, forming, assembly and storage, which can run to many months. That is its first job, and it is a corrosion job.
Its second job is to get out of the way. Plates and sections are welded and thermally cut after priming, through the primer, because stripping it back at every joint would make fabrication impossible. So a coating selected for atmospheric protection is also being selected for how it behaves when an arc passes over it.
Those two requirements pull in opposite directions. Thicker film gives better protection during storage and worse behaviour at the arc. More effective inhibitors and pigments give better protection and can give worse fume and worse weld quality. Every shop primer is a compromise, and the compromise was struck by whoever specified it, usually without the people who will blast the tank in five years being consulted.
Shop primer is specified to survive the shop and chosen to be destroyed at every weld. Buyers who treat it as a minor consumable are deciding a surface preparation scope they will not see for years.
Why There Is a Standard Just for This
The clearest evidence that weld-through is a real engineering problem rather than a nuisance is that it has a four-part international standard devoted to it.
ISO 17652-1 sets standardised conditions for assessing how shop primers affect the weldability of steel. Its stated uses are telling: supplier development and production quality control, supplier declarations of primer properties, contract references between supplier and customer, and customer acceptance testing. It also allows different primers to be compared under the same welding procedure, or different welding procedures to be compared with the same primer.
Part 1, General requirements. The standardised conditions for the testing, together with the health, safety and environmental precautions that the other parts refer back to.
Part 2, Welding properties of shop primers. How the primer affects the weld itself, which is where porosity and weld quality are assessed.
Part 3, Thermal cutting. Cutting is a separate process with separate behaviour, and the standard treats it separately rather than assuming welding results transfer.
Part 4, Emission of fumes and gases. Primers are rated on how they affect fume and gas emission during welding, which makes this an occupational health question with a test method attached.
Why it matters commercially: because Part 1 names contract references and customer acceptance testing among its uses, a buyer is entitled to ask for declared results rather than accept an assurance that a primer is weldable.
The practical reading is that weldability is a measurable, declarable property. A supplier who cannot produce declared results against this series is asking the yard to take the behaviour of the primer on trust, and the yard is the party that will absorb the rework if the trust is misplaced.
The Three Failure Modes at the Arc
What goes wrong when an arc crosses a primed surface falls into three groups, and they land on three different people.
The first is weld quality. Organic and inorganic film constituents decompose at arc temperature and release gas into the weld pool, which can produce porosity in the deposited metal. Porosity in a structural weld is a defect found by inspection and repaired by grinding out and rewelding, so the cost arrives as rework rather than as a coating problem, and is often never attributed to the primer at all.
The second is damage beyond the joint. Heat travels into the plate well past the fusion zone, and the primer in that band is degraded whether or not it looks it. That burn-back zone is coating that has to be removed and replaced during the main coating works, and its width is a function of the welding process and heat input rather than of anything the painter controls.
The third is fume. Welding through a zinc-containing primer adds the primer’s decomposition products to the welding fume, which is why fume and gas emission has an entire part of the standard series to itself. This is a ventilation and respiratory protection question, settled by the yard’s occupational health arrangements and the applicable exposure limits rather than by the coating specification, and the primer selection influences how demanding those arrangements have to be.
Porosity shows up as weld rework. Burn-back shows up as blasting. Fume shows up in the welder’s breathing zone. One decision, three budgets, and usually three different managers.
What the PSPC Does About It
For ballast tanks and the other spaces within its scope, the IMO Performance Standard for Protective Coatings turns all of this into contract language, and the provisions are unusually specific.
On type, Table 1 item 2.3 names a zinc containing inhibitor free zinc silicate based primer or equivalent, with compatibility with the main coating system to be confirmed by the coating manufacturer. On what happens next, item 3.2 is where the money sits. Where the shop primer has not passed prequalification, the requirement is blast cleaning to Sa 2, removing at least 70 per cent of the intact shop primer. Where the complete epoxy system including its shop primer did pass prequalification, intact primer may remain, provided the same epoxy coating system is used, and the retained primer is cleaned by sweep blasting, high pressure water washing or an equivalent method.
The 70 per cent provision is the one to understand before signing anything. It means the prequalification status of the shop primer, decided at the very start of fabrication, determines whether the main coating works begins with a sweep and a wash or with blast cleaning that removes most of the existing film. That is a large difference in a tank, and it is set by a decision made long before any coating contractor is appointed.
The verification requirement cuts the same way. Where retention relies on declared compatibility, the thickness and cure of the zinc silicate primer have to be confirmed against specified values, and cleanliness of the steel is monitored during shop primer application. A yard that did not record those values cannot later demonstrate that retention was permissible, so the fallback is removal.
The Decision Worth Making Early
Three questions settle most of this, and all three are cheap to ask at fabrication stage and expensive to answer afterwards.
Has the shop primer been tested and declared against the weldability series, so the yard knows what it is dealing with at the arc rather than finding out from inspection results. Has the complete coating system, including this shop primer, been prequalified together, since that single fact decides whether 70 per cent of the film comes off later. And were the application records, thickness, cure and substrate cleanliness, kept in a form that survives the handover, because retention without records is not retention.
None of these are coating questions in the ordinary sense. They are fabrication decisions with coating consequences, which is exactly why they fall between the two and get made by default. Our industrial painting and coating work covers the surface preparation that follows from them, including the burn-back and weld zones that no shop primer specification can protect.
Frequently Asked Questions
Why does welding over shop primer cause porosity?
Because film constituents decompose at arc temperature and release gas into the weld pool, which can be trapped as porosity in the deposited metal. The effect depends on the primer, its thickness and the welding process, which is why ISO 17652-2 covers the welding properties of shop primers as a testable characteristic and ISO 17652-3 treats thermal cutting separately rather than assuming welding results apply.
What shop primer does the PSPC allow for ballast tanks?
Table 1 item 2.3 of resolution MSC.215(82) names a zinc containing inhibitor free zinc silicate based primer or equivalent, with compatibility with the main coating system confirmed by the coating manufacturer. Where compatibility is declared and the primer is to be retained, inspection requires the thickness and curing of the zinc silicate primer to be confirmed against the specified values.
When does intact shop primer have to be removed?
Under Table 1 item 3.2, where the shop primer has not passed prequalification the requirement is Sa 2 removing at least 70 per cent of the intact primer. Where the complete epoxy system including its shop primer passed prequalification and the same epoxy system is used, intact primer may remain, cleaned by sweep blasting, high pressure water washing or an equivalent method. Damaged shop primer and welds require Sa 2½ in either case.
Is welding fume from shop primer a separate concern?
Yes, and it has its own test method. ISO 17652-4 rates shop primers on how they affect the emission of fumes and gases during welding, with the health, safety and environmental precautions for the testing set out in Part 1. Managing the exposure itself is a matter for the yard’s occupational health arrangements and the applicable exposure limits; the primer choice determines how demanding those arrangements need to be.
Sources: ISO 17652-1:2003, Welding, Test for shop primers in relation to welding and allied processes, Part 1, General requirements, establishing standardised conditions for assessing the effect of shop primers on the weldability of steel, for supplier development and production quality control, supplier declarations of primer properties, contract references between supplier and customer and customer acceptance testing, and for comparing primers under the same welding procedure or procedures with the same primer · ISO 17652-2:2003, Part 2, Welding properties of shop primers · ISO 17652-3:2003, Part 3, Thermal cutting · ISO 17652-4:2003, Part 4, Emission of fumes and gases, rating shop primers on their effect on the emission of fumes and gases during welding, with testing precautions in Part 1 · IMO resolution MSC.215(82), Performance Standard for Protective Coatings for Dedicated Seawater Ballast Tanks in All Types of Ships and Double-Side Skin Spaces of Bulk Carriers, Table 1 item 2.3 on shop primer type and manufacturer confirmation of compatibility, item 3.1 on weld seam treatment and a minimum 2 mm rounded edge radius, item 3.2 on Sa 2 removing at least 70 per cent of intact shop primer not having passed prequalification, retention of intact primer where the complete epoxy system passed prequalification and the same system is used, cleaning of retained primer by sweep blasting or high pressure water washing, and Sa 2½ on damaged shop primer and welds, item 3.3 on erection damage requiring St 3 or better or Sa 2½ where practicable and Sa 2½ for contiguous damage over 25 m² or over 2 per cent of total tank area, and section 6.2 on confirming the shop primer against item 2.3, confirming declared zinc silicate primer thickness and curing against specified values and monitoring steel cleanliness during shop primer application
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