Protective coating
A coating system designed for twenty years fails in three, and everyone blames the paint. In almost every case we are called to inspect, the paint was fine. Six things go wrong, and all six are preventable.
Viram Mariv Ltd · Tema, Ghana · 9 min read
Ghana's coastal industrial belt is one of the harder places in the world to keep steel protected. Salt-laden air, humidity that rarely drops, strong UV, and a wet season that puts moisture on every surface overnight. Under ISO 12944 — the international standard that classifies how aggressive an environment is — most steel in Tema, Accra and Takoradi sits in C5, the marine category, and structures in the splash zone sit in CX, the most severe category defined.
A properly specified and properly applied system in a C5 environment should run more than fifteen years before major maintenance. Heavy-duty specifications reach twenty-five. When something fails in three, it is almost never because the material was wrong.
A protective coating is not paint. It is a corrosion control system, specified as a stack, where every layer has a job:
Each coat has a stated dry film thickness, a minimum recoat time and a maximum recoat time. Miss any of those and the system does not perform as designed, whatever it says on the tin.
This is the biggest one on the coast and the least understood.
Steel that has been exposed to marine air holds chloride salts inside its corrosion pits. They are invisible. Dry blasting cleans the surface around them beautifully and can drive them deeper into the pit. The steel passes visual inspection at Sa 2½ with contamination still in it.
Then the coating goes on and the chlorides start drawing moisture through the film by osmosis. Blisters form. The coating lifts from underneath. The steel that was certified clean is corroding beneath a film that measured perfectly.
Prevention: salt testing on coastal and marine steel, and a fresh water wash where levels exceed the coating manufacturer's limit. It adds a step and it is the cheapest insurance on the whole job.
When steel is colder than the dew point of the surrounding air, moisture condenses on it. You often cannot see it. Apply a coat onto that film of moisture and adhesion is compromised before it cures.
In Ghana this catches people twice a year. During the harmattan, steel radiates heat overnight and sits cold in the morning. During the rains, humidity stays high enough that the margin closes constantly.
Prevention: substrate temperature, air temperature, relative humidity and dew point measured and recorded before every single coat. The rule is that steel must be at least three degrees above dew point. If it is not, the coat waits.
A contractor who has never once stopped work because of dew point is not measuring dew point.
Primer bonds mechanically to the anchor pattern cut into the steel by blasting. Too shallow and there is nothing to grip. Too deep and abrasive peaks stand proud of the applied film, giving you exposed high points under a coating that measured correctly on average.
Prevention: profile measured to ISO 8503 and matched to the primer's stated range, recorded by area. If the substrate was wire brushed or pressure washed rather than blasted, there is no profile at all. More on how profile works here.
Epoxies have a maximum overcoat time as well as a minimum. Once fully cured and past that window, the surface is too tight for the next coat to bond chemically.
This happens constantly on projects that stall — the build coat goes on, the job stops for three weeks over a payment dispute or a shutdown extension, then somebody applies the topcoat straight over it. Six months later you have intercoat delamination, where the coating separates cleanly between two layers.
Prevention: track recoat windows against the manufacturer's data sheet. Where the window has passed, the surface must be abraded before the next coat. That is a known, manageable step — but only if someone noticed.
Spray applies film evenly to flat surfaces and thinly over edges, welds, bolt heads, corners and cut-outs. Surface tension pulls wet coating away from a sharp edge as it flows.
Those edges are exactly where corrosion starts. Which is why specifications call for a stripe coat — brush application to every edge, weld and detail before the main spray coat goes on.
It is slow, it is unglamorous, and it is the first thing dropped when a programme is under pressure. Look at almost any prematurely failed structure and the rust will be running along the edges and welds first.
Prevention: stripe coating written into the method statement and inspected as a hold point, not left to the crew's discretion.
A specification says 260 microns total dry film. If nobody measured it, nobody knows whether it was achieved. Thin film means the barrier is thinner than designed and moisture reaches the steel sooner. Excessive film on some materials causes internal stress and cracking.
Prevention: wet film checked with a comb gauge during application, so a coat can be corrected while it is still wet. Dry film measured with a gauge after cure, per coat and for the total system, logged by area.
Ask for this at tender stage, not at handover. A contractor who intends to record it will price for it. One who does not will tell you it is unnecessary.
You do not need to be a coating inspector to tell whether a job is being run properly. A few things are visible from the ground:
Sometimes, and it is much cheaper when it is viable. The existing system has to be sound, compatible with the new material and well adhered. That needs an adhesion test and a compatibility check, not a visual judgement.
Under ISO 12944, a high-durability system in a C5 marine environment is designed for more than fifteen years to first major maintenance, with heavy-duty specifications reaching twenty-five. That assumes correct preparation, correct film thickness and inspection at every stage.
Only if the application is controlled. A premium system badly applied fails faster than a mid-range system applied properly. Spend the money on preparation and inspection first.
Yes, with containment and planning around humidity and dew point. What should never happen is applying a coat outside the manufacturer's stated conditions to keep a programme on track.
Usually. Blister patterns, delamination planes and where the corrosion started tell you a lot — edge failure points at stripe coating, blistering from beneath points at salts or moisture, clean separation between layers points at a missed recoat window.
Viram Mariv does the surface preparation and the coating, so nobody can hand the blame back and forth. Based in Tema, working across Ghana and West Africa. Send us the scope or the specification and we will come and look.
Written by the Viram Mariv team. We are an industrial surface preparation and protective coating contractor based in Tema, Ghana, working across West Africa.