Painting new steel: in the shop or in the field?

For new structural steel, the most reliable approach is usually a split one: blast cleaning and primer, sometimes an intermediate coat too, in the shop, where cleanliness and conditions are under control, then touch-up of damage, treatment of erection joints and the topcoat in the field. Painting everything in the field only makes sense when shop work is not possible or when an existing structure requires a single campaign.

The choice between shop and field is made in the specification, well before the first pieces are delivered. It determines the quality of surface preparation, the erection schedule and the amount of touch-up to expect. This guide helps plant owners, project managers and maintenance managers in Quebec and Ontario decide, alongside our steel structure painting page and our reference page on protective coatings.

Why does the shop give better surface preparation?

Because it brings together the conditions that make good preparation: automated or booth blasting, controlled abrasive, stable temperature and humidity, and access to every face before erection. The same results are possible in the field, but they take more effort and more containment.

In the shop, each piece is isolated and accessible on all sides. Blasting consistently reaches the required grade (for example SSPC-SP 6 or SP 10, depending on the selected system) and the specified profile. The primer is applied right after, before recontamination. Thickness is measured on each piece and recorded before shipping.

In the field, the erected structure hides faces, scaffolding and aerial lifts limit access, and blasting requires containment of the debris. Cold weather and humidity shorten the work windows, as our article on cold-weather coating application explains.

How do shop, field and split approaches compare?

The comparison rests on preparation, handling, schedule and inspection. The table below summarizes the trends; the project specification and the system data sheet set the actual requirements.

The paint system itself is selected from the corrosivity of the environment; our guide to ISO 12944 C3, C4 and C5 paint systems helps define it.

How do you limit handling damage and organize touch-up?

You limit damage by respecting the dry time before handling, protecting lifting points and storing pieces on dunnage; you organize touch-up by writing it into the specification, with a defined method, product and inspection.

• Dry and cure before shipping. The primer must reach the hardness stated on the data sheet before loading. A piece handled too early keeps sling and dunnage marks.
• Lifting and transport. Padded slings, protected dunnage, separation between pieces and lifting points defined in advance.
• Storage on site. Pieces raised off the ground, tilted to shed water, away from mud and de-icing salt.
• Touch-up. Damaged areas are cleaned to the specified grade, often by power tool cleaning per SSPC-SP 3, SP 11 or SP 15 (see our article on power tool cleaning), with feathered edges to blend into the existing film.
• Touch-up product. The same primer, or a compatible product named in the specification and the data sheet.

SSPC-PA 1 covers shop, field and maintenance coating application, including touch-up. It often serves as the framework for the specification.

What should be done at erection joints and field welds?

Areas that will be welded or bolted in the field are handled separately: they are masked or receive only a compatible primer, then they are prepared and painted after assembly. The specification should identify them on the drawings.

• Field welds. An unpainted strip, or one coated with a weldable primer per the data sheet, is left on each side of the joint. After welding, slag and spatter are removed, the area is prepared and the full system is applied.
• Slip-critical bolted connections. Faying surfaces are left unpainted or receive a coating qualified for the required slip coefficient, per CSA S16 and the RCSC specification. An arbitrary primer can let the connection slip.
• Ordinary bolted connections. Bolts, nuts and washers need their own protection; our article on bolt and connection corrosion covers the options.
• Zinc-rich primer. Common in the shop; its compatibility with touch-up and topcoat must be checked, as our inorganic vs organic zinc primer comparison explains.

When should the topcoat be applied in the field?

Apply the topcoat in the field when appearance matters, when erection damage would be extensive or when the time between shop and erection risks exceeding the primer's recoat window. Otherwise, a shop topcoat remains possible, with touch-up.

The critical point is the maximum recoat interval. A primer left outdoors too long hardens at the surface, chalks or becomes contaminated, and the topcoat bonds poorly. When the interval is exceeded, the data sheet prescribes cleaning, scuff sanding or a tie coat. Our article on pot life and recoat windows explains how to read these limits.

Before the field topcoat, the surface is washed to remove the dust, grease and salts that build up during erection. Ambient conditions (surface temperature, dew point, relative humidity) are measured and recorded at every session.

Which inspections should be planned in the shop and in the field?

Plan an inspection at every stage and a single record that follows the pieces from the shop to the site. Without that record, it becomes impossible to tell whether a defect comes from preparation, handling or the topcoat.

• In the shop. Cleanliness and profile, ambient conditions, dry film thickness per SSPC-PA 2, appearance, piece identification and application dates.
• On receipt at the site. Visual inspection for damage, photo record, report to the supplier.
• After touch-up and joints. Preparation grade achieved, thickness of repaired areas, proper overlap onto the existing film.
• After the topcoat. Total thickness, appearance and adhesion where the specification requires it.

ISO 12944-7 covers the execution and supervision of paint work. Our article on coating inspection details the measurements, and our guide to industrial painting specification contents shows where to write these requirements.

Frequently asked questions

Can the whole system be applied in the shop to save time in the field?

Yes, but plan for topcoat touch-up after erection, which is often visible. For a structure where appearance matters or one exposed to a severe environment, a field topcoat gives a more uniform result.

Does damaged shop primer need to be blasted again?

Not usually. Damaged areas are prepared locally to the grade set in the specification, often by power tool cleaning, then receive the touch-up primer. Abrasive blasting is only required if the specification calls for it.

Why leave unpainted strips next to field welds?

Because welding heat burns the coating and gives off fumes, and some primers affect weld quality. The strip is prepared and painted after welding.

Who is responsible for transport and erection damage?

The specification decides. It should state who touches up, with which product, to which preparation grade and who inspects, otherwise the damage is left without an owner.

How do you get a quote for new steel?

Our crews and qualified subcontractors handle preparation, touch-up, joint treatment and field topcoats on new steel frames as part of our industrial painting services, including in Montreal, Montérégie and Ontario. Send the drawings, tonnage or surface areas, the planned paint system and the erection schedule through the request a quote form: pricing on consultation, reply within 24 business hours.