Surface Finish Systems for Structural Steel: Primer, 2K Paint and Hot-Dip Galvanising
Finish is usually the last line agreed on a heavy frame and one of the first things to fail in service. It is also the only part of the specification that changes the design, galvanising in particular imposes geometry requirements that are painful to discover after the drawing is released.
Surface preparation comes first
Whatever goes on top, the steel underneath has to be prepared. This is not a detail; it is most of what determines whether the coating lasts.
Abrasive blast grade (ISO 8501-1):
| Grade | Description | Typical use |
|---|---|---|
| Sa 1 | Light brush blast | Rarely adequate |
| Sa 2 | Thorough blast | Interior, mild exposure |
| Sa 2½ | Very thorough, the workhorse | Standard for exported equipment |
| Sa 3 | Blast to visually clean steel | Severe service, immersion |
Sa 2½ is the sensible default. Specify it explicitly; “blast and paint” without a grade means whatever the shop’s standard is.
Also specify surface profile (roughness), typically 40–75 µm for most paint systems, and require blasting and priming within four hours in a humid climate, flash rust on a blasted surface undoes the work.
Mill scale is the enemy. It is a hard, brittle oxide that looks fine and disbonds in service, taking the coating with it.
The three systems
1. Blast and shop primer
Sa 2½ blast, then a single primer coat, typically a zinc-rich epoxy or an inorganic zinc silicate at 50–75 µm.
- Cost: lowest
- Life: months of protection, not years
- Use it when: the frame is indoors, or will be top-coated by someone else, or is a sub-assembly that gets coated after final assembly
It is a holding coat. Do not treat it as a finish.
2. Blast, primer and 2K industrial topcoat
The standard system for exported equipment. Typically a zinc-rich epoxy primer, an epoxy mid-coat where thickness is needed, and a polyurethane topcoat for UV stability and colour.
Total dry film thickness commonly 180–320 µm depending on the corrosivity category (ISO 12944 C3 to C5).
- Cost: moderate, typically 5–12% of the job
- Life: 10–15 years in C3, less in C5 marine
- Use it when: you need a specific colour, the frame is indoors or in moderate exposure, or the geometry rules out galvanising
Specify the ISO 12944 corrosivity category and durability class rather than a paint brand. That gives the shop a performance target it can meet with locally available materials, which is faster and cheaper than importing a specific product.
3. Hot-dip galvanising
Immersion in molten zinc at roughly 450 °C, forming metallurgically bonded zinc-iron layers. Typical coating 85 µm or more on structural thicknesses (see ISO 1461 for the thickness table by section).
- Cost: highest per square metre, but often lowest over life
- Life: 25 years or more in C3, well over a decade in C5
- Use it when: the frame is outdoors, in wet process, or maintenance access will be poor
And here is what it does to your design.
The design consequences of galvanising
This is the part that catches people.
Vent and drain holes are mandatory
Every closed section must have holes so that air escapes on immersion and zinc drains on withdrawal. A sealed hollow member taken into a 450 °C kettle with trapped moisture is an explosion hazard, galvanisers will simply refuse the part.
Hole sizing follows the galvaniser’s rules (typically related to the section area, with a stated minimum diameter), and holes must be at the ends and diagonally opposite so the section fills and drains fully. Put them on the drawing rather than leaving the galvaniser to drill them where it suits.
Distortion risk
Immersion heats the part rapidly and unevenly. Thin sections, long unstiffened plates and assemblies of very different thicknesses can move noticeably. Mitigations: symmetrical design, avoid mixing 6 mm and 40 mm in one assembly, and stress relieve before galvanising.
Galvanising is not stress relief, the kettle is well below the soak temperature and the dwell is minutes, not hours. See stress relief for welded machine bases.
Kettle size is a hard limit
Kettles have finite length, width and depth. A piece longer than the kettle can sometimes be double-dipped, which leaves a visible line and doubles the thermal cycling. A piece wider or deeper than the kettle cannot be galvanised at all.
Ask for kettle dimensions early, alongside crane capacity and oven length, see how to choose a heavy fabrication supplier.
Machined surfaces do not survive it
Zinc build-up is uneven and thick by machining standards. Any face with a tolerance must be masked and machined afterwards, or excluded from the dip. Threads need to be tapped oversize or chased after galvanising.
If your frame has machined mounting pads and needs galvanising, the sequence is: fabricate, stress relieve, galvanise, then machine the pads. Say so on the drawing.
It can be painted over
Duplex systems, galvanising plus paint, give the longest life of anything on this list and allow a colour. They need a suitable surface treatment (sweep blast or an etch/T-wash) for adhesion. Expensive, and appropriate for severe marine service.
Quick comparison
| Shop primer | Blast + 2K paint | Hot-dip galvanising | |
|---|---|---|---|
| Relative cost | 1 | 2–3 | 3–5 |
| Life in C3 | Months | 10–15 yrs | 25+ yrs |
| Colour options | No | Any | Grey only, unless painted over |
| Edge protection | Poor | Poor, edges thin | Excellent |
| Damage repair | Easy | Easy | Zinc-rich paint, inferior to original |
| Size limit | None | None | Kettle size |
| Design impact | None | Minimal | Vent holes, distortion, machining sequence |
Note the edge row. Paint pulls thin on sharp edges and corners, which is where corrosion starts. Specify edge grinding to a 2 mm radius, or stripe coating on edges and welds, if you are painting a frame for outdoor service.
What to write on the drawing
- Surface preparation: “Abrasive blast to Sa 2½ per ISO 8501-1, profile 50–75 µm”
- System and performance target: “Coating system to ISO 12944 C4 High durability” or “Hot-dip galvanise to ISO 1461”
- Dry film thickness: total and per coat
- Colour: with a RAL number
- Masking: listing machined surfaces and threads excluded
- Inspection: DFT readings, adhesion test, and who signs
Then ask for the DFT report as a deliverable. It is a small document and it is the only evidence the coating is the thickness you paid for.
Not sure whether your frame should be painted or galvanised, or whether it even fits a kettle? Send the drawing and the service environment and we will tell you within 24 business hours.
See also: ISO 12944 for painted steel frames, on categories, durability and inspection.