Zinc-rich vs Epoxy vs PU
Choose by mechanism + job

Three distinct corrosion protection mechanisms: • **Zinc-rich** = sacrificial anode (galvanic) — zinc dust ≥ 80% (ASTM D520) corrodes in place of steel. Electrons flow from zinc → steel becomes cathode (does not corrode). Continues to protect even when coating is scratched • **Epoxy** = barrier protection — impermeable film blocks H₂O / O₂ / chloride from reaching steel. If film breaks, protection ends at that point • **Polyurethane (PU)** = barrier + UV stability — gloss retention 5-10 years, withstands UV degradation that epoxy cannot ISO 12944 C5 system uses all three: primer + intermediate + topcoat.

Per ISO 12944-1:2017 H (high) durability: • **C3 Medium environment** — 3-coat 220µm DFT → 15-20 years • **C4 High** — 3-coat 320µm DFT → 15-25 years • **C5 Very High (marine + industrial)** — 3-coat 400µm DFT → 15-25 years We do not sell this 3-coat system — the anti-rust coatings we sell are SGP-001 (one-part PU, goes over surface rust without blasting) and SGU-201 (epoxy for wet or underwater steel); see Industrial coatings (/en/paint).

Single-layer (standalone, not in a system): • Zinc-rich primer alone: 1,000-2,000 hr before first red rust • Epoxy standalone: 500-1,000 hr (depends on formulation) • PU standalone: 200-500 hr (not primarily a corrosion barrier) In-system (3-coat zinc + epoxy + PU): 3,000-5,000 hr before blistering / undercutting Thai government TOR typically requires ≥ 2,000 hr salt spray. Petrochemical projects ≥ 4,000 hr.

Not recommended — chemistry reasons: • PU undergoes hydrolysis with continuous water contact — fine as a topcoat (away from steel), but as a middle coat next to primer, accumulated moisture causes delamination • Epoxy intermediate builds 100-200µm per coat → PU builds only 50-80µm = more coats needed • Chemical resistance: epoxy withstands acid + alkaline + solvent. PU degrades in strong acid • Cost: epoxy is 20-30% cheaper than PU Standard international practice: epoxy as barrier middle coat, PU as UV-resistant topcoat — don't swap.

We do not sell zinc-rich primer or high-build epoxy intermediates. The anti-rust coatings we sell, with prices on the site: • **SGP-001** one-part PU ฿8,100 per 1 US Gallon • **NOVA Rust Shield** ฿5,070 per 1 US Gallon • **SGU-201** epoxy for wet or underwater steel, from ฿2,560 per Size S (A 1kg + B 0.2kg) • **NOVA Aqua Shield** from ฿1,000 per Set S (A 1.0kg + B 0.2kg) unit price, excluding 7% VAT · nationwide delivery · specs on Industrial coatings (/en/paint)