Marindec PaintsStructural steel corrodes from the surface in, so the coating has to keep water and oxygen off the metal for years. That takes three things working together: a clean surface, a primer matched to the steel, and enough topcoat film. Skip any one and the system fails early, usually at edges, welds and bolt holes.
Step 1: Remove millscale, rust and oil
New hot-rolled steel carries millscale, a brittle oxide skin that eventually flakes off and takes the paint with it. For exterior and industrial steel, solvent clean and grit blast to Sa 2½ (near-white metal). For interior steel in dry conditions, solvent clean plus wire brushing or grinding is the minimum. Full method table: surface preparation guide.
- Paint only when the steel is at least 3°C above the dew point. This is a condition on every Marindec datasheet.
- Prime blasted steel the same day, before flash rust appears.
Step 2: Prime
The primer carries the corrosion protection and gives the topcoat something to grip. For most structural steel, use FPW15 Fast Dry Primer or the anti-corrosive ZPPW15: apply 20–35 µm dry film, touch dry in about 10 minutes, overcoat within 2 hours. Galvanised steel needs Galvanised Iron Primer instead. Red oxide primers lose adhesion on zinc.
Step 3: Topcoat
| Environment | System | Why |
|---|---|---|
| Interior steel, workshops, dry plant | FPW15 primer + Fast Dry Enamel | Fast turnaround: enamel touch dry ~10 min, 20–45 µm DFT |
| Exterior steel, gates, fencing, plant | ZPPW15 primer + AGRIP30+ High Build | Higher build and chemical resistance; AGRIP30+ can also be used without a primer for lighter duty |
| Exposed steel needing a long-life finish | 2K primer + 2K Polyurethane | Aliphatic PU, very good weathering; needs a 2K polyurethane or epoxy primer, not an alkyd |
| Hot steel: flues, exhausts, ovens | Heat Resistant Aluminium | Silicone resin finish for continuous dry heat up to 400°C |
Step 4: Check film thickness
Datasheet coverage figures are theoretical. Real-world coverage is 50–70% lower, depending on the steel profile and the application method. Use a wet film gauge while painting and stripe-coat edges, welds and bolts by brush first. These are where films are thinnest and corrosion starts. See the film thickness guide.
Common mistakes
- Painting over millscale: the coating fails when the scale lets go.
- Alkyd primer under a 2K polyurethane topcoat: the solvents can lift the primer.
- Over-thinning to make paint go further: datasheets advise little or no thinning, and excess thinner reduces film build and sag resistance.
- Overcoating outside the window: fast dry enamels can wrinkle if recoated at the wrong time.
For a full specification for your steel, send us the details. See also: anticorrosive coating systems.