What to check on a Shop Primer (Pre-Construction).
Steel arrives at a shipyard as raw plate, gets blast-cleaned to a defined surface profile, and then sits in the stockyard and moves through cutting, forming and welding before it becomes part of a block. Bare blasted steel starts rusting again within hours in a humid yard, so it is immediately coated with a thin shop primer — typically 15 to 20 microns dry film thickness — thin enough not to interfere with welding and cutting, and fast-curing enough to keep the production line moving. This is fundamentally different from…
What sets this coating apart
Steel arrives at a shipyard as raw plate, gets blast-cleaned to a defined surface profile, and then sits in the stockyard and moves through cutting, forming and welding before it becomes part of a block. Bare blasted steel starts rusting again within hours in a humid yard, so it is immediately coated with a thin shop primer — typically 15 to 20 microns dry film thickness — thin enough not to interfere with welding and cutting, and fast-curing enough to keep the production line moving. This is fundamentally different from the ship's final coating system applied after block construction: shop primer is a temporary holding coat, not a corrosion protection system meant to last the ship's service life, and in way of weld seams it is typically removed or is thin enough to burn through cleanly during welding.
Build-up: primer chemistry and application
Zinc silicate primers
An inorganic zinc silicate primer gives strong weld-through performance and good long-term corrosion resistance if the steel ends up staying primed longer than planned, but it needs a properly controlled cure (moisture and temperature) and, if overcoated later, needs a light sweep blast or thorough cleaning first because zinc silicate's inorganic surface does not always bond well to a topcoat straight off the stockyard.
Zinc epoxy primers
An organic zinc-rich epoxy primer is more forgiving to apply and recoat, tolerates a wider humidity window during cure, but generates more weld fume and porosity risk if applied thicker than specified, since the resin content burns rather than vaporising as cleanly as an inorganic binder.
Application
Most yards run an automated primer line directly after the blast booth, spraying a controlled, even film across the plate in one pass; DFT is checked with a magnetic gauge at intervals along the line, because both an under-thick film (no real protection) and an over-thick film (weld porosity, fume) are rejects.
Typical faults
| Fault | Consequence |
|---|---|
| Primer applied thicker than specified | Weld porosity and increased fume during cutting and welding, sometimes requiring rework of the weld |
| Zinc silicate primer overcoated without a sweep blast | Poor adhesion of the final coating system, showing up later as blistering or flaking in way of primed steel |
| Extended stockyard exposure between priming and block erection | Rust bloom breaking through the thin primer film, especially at plate edges and in humid or coastal yards |
| Arc strikes or mechanical damage during handling | Local bare steel exposed well before the block reaches final coating, starting corrosion at a point the topcoat then has to bridge |
What to look for in a supplier
- Compatibility testing against the yard's specified final coating system, ideally referencing the IMO Performance Standard for Protective Coatings (PSPC) compatibility requirements where the primer sits under a ballast tank coating
- Consistent DFT control and weld-through performance data, not just a datasheet figure
- Realistic recoating window before the primer needs a sweep blast, matched to the yard's typical time between priming and block erection
- Two-component product shelf life suited to the yard's storage and mixing turnaround
A block that has sat primed in the stockyard far longer than planned should be checked for rust bloom before it goes to final coating; primer that looked sound on the day it was sprayed does not stay that way through a delayed production schedule.