Surface preparation
Most coating failures are blamed on the paint. Most of them start with the blast. Here is what the grades mean, why the profile matters more than the finish looks, and how to read a specification before you price the job.
Viram Mariv Ltd · Tema, Ghana · 8 min read
Somebody hands you a specification that says the steel must be blasted to Sa 2½ with a 50 to 75 micron profile. If that sentence means nothing to you, you are not unusual — plenty of people commissioning industrial coating work in Ghana have never had it explained. And that gap is expensive, because those two numbers decide whether the coating lasts twenty years or three.
This is what happens when you blast steel, what the grades actually describe, and how to tell whether the work you paid for was done.
Compressed air drives abrasive through a nozzle at high velocity. On impact it does two things at once, and only one of them is obvious.
The obvious one is removal. Rust, mill scale, old coating, dirt and contamination get stripped off. The steel comes up clean and grey.
The second is the one that matters more. Every abrasive particle that hits the surface cuts a small crater. Millions of them together produce a texture — a pattern of peaks and valleys called the anchor profile. Primer flows into that texture and locks on mechanically. Without it, primer is sitting on a smooth surface with nothing to grip.
A coating on unprofiled steel can look flawless on handover and peel off in sheets eighteen months later. The failure was designed in on day one.
This is why "we cleaned it thoroughly" is not the same as "we blasted it." Wire brushing, grinding and pressure washing all remove contamination. None of them cut a proper anchor profile.
Surface cleanliness is graded under ISO 8501-1. The grades describe how much contamination is allowed to remain, and they are assessed visually against photographic standards.
| Grade | What it means | Typically used for |
|---|---|---|
| Sa 1 | Light blast. Loose scale and rust removed only. | Temporary protection, low-duty work |
| Sa 2 | Commercial blast. Most contamination gone, some staining allowed. | Mild environments, some maintenance coatings |
| Sa 2½ | Near-white. Only light shadows or streaks remain. | The workhorse. Most epoxy and zinc-rich primers |
| Sa 3 | White metal. No visible contamination at all. | Immersion service, tank linings, severe chemical exposure |
If a specification names an SSPC or NACE grade instead, the rough equivalents are SP 6 for Sa 2, SP 10 for Sa 2½ and SP 5 for Sa 3.
Sa 2½ is what nearly every industrial and marine specification in Ghana calls for. It is the grade almost all heavy-duty primers are formulated around. Going to Sa 3 costs meaningfully more in time and abrasive, so it is reserved for tanks and immersion work where nothing less will hold.
Cleanliness gets checked because it is visible. Profile gets skipped because it needs a gauge.
Profile is measured under ISO 8503 and stated in microns — the average height between peaks and valleys in the blasted texture. Every primer has a range it was designed for, usually somewhere between 40 and 85 microns.
Both ends of the range matter:
Profile is set by abrasive size and shape, not by how long you blast for. Blasting longer with fine media makes the surface cleaner, not rougher. If your specification calls for a deeper profile, the abrasive has to change.
Abrasive is usually the largest consumable cost on a blasting job, and the choice affects speed, profile and how much waste you are left disposing of.
Copper slag is the common expendable media across West Africa. It cuts fast, gives a good angular profile and is widely available. Single use — once it has hit the steel it is spent, so a large job generates a lot of waste to bag and remove.
Garnet costs more per tonne but can be recovered and reused several times, and it produces far less dust with very low free silica. Where health exposure or containment is a concern, it is often cheaper overall despite the headline price.
Steel grit and shot are recycled continuously in enclosed setups. Angular grit cuts profile, round shot peens and cleans without cutting. Efficient at volume, but the recovery equipment only makes sense in a fixed facility or on a large project.
Aluminium oxide is hard, sharp and long-lasting, used where the substrate is tough or the work is precise. Glass bead gives a soft satin finish without cutting a profile, so it suits cosmetic cleaning of stainless and alloy rather than coating preparation.
What we do not use is dry silica sand. Respirable crystalline silica causes silicosis, and the risk is not manageable at the scale industrial blasting operates. Most international specifications now exclude it outright, and any contractor still using it on open sites is telling you something about how they run everything else.
Freshly blasted steel is chemically bare and highly reactive. In Ghana's humidity, especially near the coast, it will flash rust within hours. On a bad day, within one.
That rust is not cosmetic. Prime over it and you have bonded the coating to a layer of corrosion product rather than to steel.
Which means blasting and priming are a single operation, not two stages. Blast only what can be primed before conditions turn or the shift ends. A contractor who blasts a whole structure on Monday and starts priming on Wednesday has wasted Monday.
Dry blasting is fast, effective and generates a large quantity of airborne dust and spent abrasive. On an open site with room to contain it, that is fine. In other places it rules the method out entirely:
That last one is worth dwelling on. Dry blasting does not remove soluble salts sitting in corrosion pits — it can drive them deeper. Those chlorides then pull moisture through the finished coating and blister it off from beneath. On badly pitted coastal steel, washing with water is the only reliable answer. Wet and hydro blasting cover both problems.
They are the same process with different media. Sandblasting is the everyday name, grit blasting uses angular abrasive to cut a profile, shot blasting uses round media that peens rather than cuts. What matters in a specification is the cleanliness grade and the profile achieved, not the label.
It varies enormously with abrasive, nozzle size, compressor output and how corroded the steel is. Clean new fabrication moves fast. Heavily pitted marine steel taken to Sa 2½ moves slowly. Any contractor quoting a general rate without seeing the steel is guessing.
If the existing system is sound, compatible and well adhered, a maintenance overcoat after cleaning and abrading can be the right call and much cheaper. That decision needs an adhesion test, not a look.
The contractor should measure and record it as part of normal work. On larger contracts a third-party coating inspector verifies independently. Either way, ask for the readings — if nobody measured, nobody knows.
Viram Mariv is based in Tema and works nationwide, including Accra, Takoradi and the Tarkwa mining belt. Send us the scope or the coating specification and we will come and look. Quotations are free.
Written by the Viram Mariv team. We are an industrial surface preparation and protective coating contractor based in Tema, Ghana, working across West Africa.