Environment classes compared

ISO 12944 C3 vs C4 vs C5
Choose the coating system by the real environment

Corrosivity category → total DFT → durability years → installed cost — three steps to a coating system that matches spec. Full ISO 12944-1 to -5 + the Sahawatthanakit industrial coating system in Thailand

Calculate 20-year TCO Request a coating system quotation

In short — which class are you?

Run an atmospheric corrosion-rate survey (12-month steel coupon test) or pick the closest environment from the lookup below

Suburban / inland industrial

C3 — Medium

Suits small workshops, warehouses, dry interiors
Corrosion rate 25-50 µm/yr — moderate threat
3-coat system 220 µm DFT = 15-25 years (H bracket)
Surface prep Sa 2 (commercial blast)

Best for

Urban factories, goods staging yards, dry warehouses, suburban steel structures

Industrial estate / coastal moderate

C4 — High

Industrial estates (Map Ta Phut, BangPoo) + moderate-salt coastal
Corrosion rate 50-80 µm/yr — significant threat
Zinc-rich primer + 3-coat system 320 µm = 15-25 years
Surface prep Sa 2½ (near-white blast)

Best for

Industrial estates, refineries, chemical tanks, structural steel within 5 km of the sea

Heavy industrial OR coastal

C5 — Very High

C5-I: heavy SO₂ industrial · C5-M: marine/offshore
Corrosion rate 80-200 µm/yr — severe threat
Zinc-rich heavy + 400 µm system = 15-25 years (peak end)
Surface prep Sa 2½ to Sa 3 (white metal blast)

Best for

Offshore platforms, marine piers, refinery stacks, foundries, splash zones

Full spec comparison table

Per ISO 12944-1, -2, -4, -5 (2017-2019) + SSPC-SP standards + the Sahawatthanakit industrial coating system

Parameter	C3 (Medium)	C4 (High)	C5 (Very High)
Corrosion rate of carbon steel	25-50 µm/yr	50-80 µm/yr	80-200 µm/yr
Typical environment	Urban, suburban industrial, inland	Industrial estates, moderate-salinity coastal	C5-I heavy SO₂ industrial · C5-M heavy coastal/offshore
Thailand example	BKK suburbs, inland factories	Map Ta Phut, BangPoo, BangPlee	Pattaya/Rayong shoreline, Map Ta Phut chimney zones
Min. surface prep (ISO 8501-1)	Sa 2 (commercial blast)	Sa 2½ (near-white)	Sa 2½ to Sa 3 (white metal)
Recommended system DFT (High durability)	220 µm (3-coat)	320 µm (3-coat zinc-rich)	400 µm (3-coat zinc-rich heavy)
Primer	Epoxy 80 µm	Zinc-rich epoxy 80 µm	Zinc-rich epoxy 80 µm
Intermediate	Epoxy 80 µm	Epoxy 160 µm	Epoxy 240 µm
Topcoat	PU 60 µm	PU 80 µm	PU 80 µm
Durability (H bracket, ISO 12944-1)	15-25 years	15-25 years	15-25 years (peak end)
Industrial coating system (Sahawatthanakit)	1-part PU primer + 2-part epoxy topcoat	Zinc-rich PU primer + 2-part epoxy, UV-stable PU	Zinc-rich heavy PU primer + 2-part epoxy + photoluminescent polyurethane (option)
Approx. installed cost (steel structure)	฿250-400/m²	฿450-650/m²	฿650-950/m²

Source: ISO 12944-1 to -5 (2017-2019), Thailand market reference prices 2026

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FAQ

Common Questions

Q1.What's the difference between ISO 12944 C3, C4, and C5?+

ISO 12944-2:2017 classifies environments by atmospheric corrosion rate on steel: • C3 (medium) — urban/inland industrial with moderate SO₂ — 25-50 µm/yr • C4 (high) — industrial + coastal with moderate salinity — 50-80 µm/yr • C5 (very high), split into C5-I (industrial heavy SO₂) and C5-M (marine heavy salinity) — 80-200 µm/yr Choose by actual climate: suburban BKK factory = C3, Map Ta Phut industrial estate = C4, near coastline Pattaya/Rayong = C5-M.

Q2.What are Durability L / M / H / VH brackets?+

ISO 12944-1:2017 defines durability as period to first major maintenance: • Low (L) < 7 years • Medium (M) 7-15 years • High (H) 15-25 years • Very High (VH) > 25 years Note: this is not a warranty — it's expected durability under normal use. Most Thai government TOR work requires H minimum.

Q3.What coating system is recommended for C3-C4?+

Per ISO 12944-5:2019 — C3 Medium: epoxy primer 80µm + epoxy intermediate 80µm + PU topcoat 60µm = 220µm DFT. C4 High: zinc-rich epoxy primer 80µm + epoxy 160µm + PU topcoat 80µm = 320µm DFT. C5 High: zinc-rich primer 80µm + epoxy 240µm + PU topcoat 80µm = 400µm DFT. What we sell for this work is SGP-001, a one-part moisture-curing polyurethane that serves as primer, mid-coat and topcoat from one can (ASTM B117 salt spray 2,000 h at 150 µm) — we do not sell zinc-rich primer.

Q4.Why does surface preparation matter so much?+

ISO 12944-4:2017 defines surface prep grades (Sa/PSt/St). C4-C5 requires Sa 2½ (near-white blast) — clean to bright steel. Skipping this causes 50-70% of paint failures. We use ISO 8501-1 visual standard + SSPC-SP 10 + ICRI CSP 3-4 for concrete. Sample surface prep report available on request.

Q5.How much better is a multi-coat system vs single thick coat?+

Single thick coat 320µm: H durability ~15-20 yr at C3. 3-coat system 320µm: ~20-30 yr at C4 (better adhesion + crack resistance). KTA/NACE methodology: 3-coat TCO is ~30-40% lower over 25-year amortization because recoat cycles are fewer. Use /tools/paint-tco-calculator to compare for your project.