Sun, salt and inspection cameras are unforgiving
Above-ground pipework, exposed risers, refinery off-plot piping and architectural steel all share one problem: they're constantly visible. Coatings that chalk under UV, fade in colour, abrade in wind-driven sand or crack with thermal cycling fail visibly — long before they fail structurally — and they get replaced years before the underlying steel is actually compromised.
Polyurethane (PU) coatings solve that. Aliphatic PU formulations are inherently UV-stable: they don't chalk, don't yellow, don't fade. The cured film is tough yet flexible enough to handle thermal expansion and minor structural movement without cracking. And it can be supplied in any RAL or BS colour you specify.
Where we recommend PU
- Above-ground refinery and pipe-rack piping — topcoat over an FBE or epoxy primer system
- Risers and exposed pipework on offshore platforms and jetties
- Field-joint coatings in PU-based heat-shrinkable sleeves for fast-cure applications
- High-abrasion exposed sections — river crossings, road crossings, rocky exposed lengths
- Architectural and decorative steel — handrails, columns, exposed plant equipment
- Coastal and offshore topsides in C5-M atmospheric corrosion zones
Why aliphatic PU specifically
The "aliphatic" qualifier matters. Aromatic polyurethanes are cheaper but degrade visibly under UV; aliphatic versions are formulated specifically to resist sunlight-driven chalking. Every PU topcoat we apply for outdoor service is aliphatic. We don't make exceptions.
Build characteristics
| Property | Value |
|---|---|
| DFT (topcoat) | 80-200 µm |
| DFT (high-build) | 250-500 µm |
| Service temperature | -40 to +120 °C dry; 60-80 °C wet |
| Dry to touch | 30-90 min |
| Recoat window | 4-24 hours |
| Pot life @ 25 °C | 2-4 hours |
| Pull-off adhesion | ≥ 6 MPa |
| Gloss (60° meter) | up to 90 |
How it's built
Part A: hydroxyl-functional acrylic or polyester polyol. Part B: aliphatic isocyanate (HDI or IPDI) hardener.
Solvent-borne, water-borne and high-solids variants available depending on your VOC limits.
Recommended above-ground system
For exposed pipeline projects we typically specify a 4-coat system:
- Surface prep — Sa 2½ abrasive blast
- Primer — zinc-rich epoxy (60-80 µm)
- Mid-coat — high-build epoxy (150-200 µm)
- Topcoat — aliphatic PU (50-100 µm) in client RAL
Total system thickness 300-400 µm, 25+ year design life in C5-M coastal environments.
Standards we apply to
- ISO 12944-5 — protective paint systems (C5 environments)
- NORSOK M-501 — offshore protective coating
- AWWA C222 — PU for buried and submerged steel water pipe
- ISO 4628 — degradation evaluation (chalking, blistering, cracking)
Common technical questions
Why pick PU over an epoxy topcoat for outdoor work? Epoxies chalk and lose colour under UV — they're great primers and tank linings, but as outdoor topcoats they look weathered within months. Aliphatic PU stays cosmetically stable for decades.
Is PU compatible with cathodic protection? Yes — PU topcoats are CP-compatible when applied over a properly designed primer/intermediate system. We've delivered CP-protected systems for marine and pipeline clients across the GCC.
Can we use PU on buried pipelines? Standard PU isn't ideal for buried service — FBE or 3LPE remains the preferred choice. However, AWWA C222-grade elastomeric PU is qualified for submerged and buried use specifically in water pipelines.
