Rust treatment methods compared
Rust Converter vs Sandblast vs Epoxy Primer
Choose by TOR + budget + service life
Three ways to treat rust on structural steel — chemical (tannic/phosphoric acid), mechanical (SSPC SP10 / ISO Sa 2.5), coating (zinc-rich epoxy + PU). Full spec table + price per m² + C3-C5 service life
In short — which method?
Decide on 3 factors — the TOR standard, budget per m², required service life. Sandblast + epoxy = the default for government TORs. Rust converter for touch-ups only. Zinc-rich primer for outdoor exposure beyond 5 years
Tight budget / hard to reach
Rust Converter
- Only for thin rust < 100 µm
- Lowest cost ฿80-180/m²
- 3-5 year life — needs a top coat
- Most government TORs do not accept it
Best for
Maintenance touch-ups on roofs, bridges that cannot be closed, emergency budgets
STANDARD (government TOR)
Sandblast + Epoxy
- SSPC-SP10 / ISO Sa 2.5 — removes 95%
- Anchor profile 50-100 µm
- Must prime within 4 hr to prevent flash rust
- 2-3 coat system lasts 15-25 years
Best for
Government TORs (SRT/DOH/EGAT/PEA), new structures, major re-coats
Cathodic Protection
Zinc-Rich Primer
- Zinc ≥ 80% — sacrificial protection
- Needs a top coat to prevent UV bloom
- C5-M service life 15-25 years
- ORAFON ฿180-240/m² (75 µm DFT)
Best for
Offshore, petrochem, transmission towers, DOH bridges, exterior > 5 years
Full spec comparison table
| Item | Rust Converter | Sandblast (SSPC-SP10) | Epoxy Primer (Zinc-Rich) |
|---|---|---|---|
| Method | Chemical — tannic/phosphoric acid reacts with Fe₂O₃ | Mechanical — abrasive (garnet/copper slag) at 6-8 bar | Coating — applied after surface prep, cures in 24-48 hr |
| Rust removal | Converts Fe₂O₃ → iron tannate / phosphate (no stripping) | Removes rust + scale + old paint 95-100% | Does not remove — coats over to stop further rusting |
| Key standards | No direct ASTM/SSPC — per the manufacturer's MSDS | SSPC-SP6/SP10/SP5 · NACE No. 1-3 · ISO 8501-1 Sa 2/2.5/3 | ASTM D520 Type II · SSPC-Paint 20 · ISO 12944-5 |
| Surface profile (anchor pattern) | None — existing surface | 35-100 µm (Testex tape, ASTM D4417) | Needs an existing 50-75 µm profile (from sandblasting) |
| Time before overcoating | 24-72 hr (fully dry + rinse residual acid) | ≤ 4 hr (prime before flash rust sets in) | 8-24 hr at 25°C (recoat window) |
| Price per m² (Bangkok 2026) | ฿80-180 (material) + labour ฿100-200 | ฿180-380 (incl. 50 µm profile, Sa 2.5) | 2-coat system ฿250-380 · 3-coat ฿380-580 |
| Service life C3 (industrial urban) | 3-5 years + must be top-coated | 0 years (must prime within 4 hr) | 8-12 years (zinc-rich 2-coat) / 15-20 years (3-coat) |
| Service life C5-M (offshore/petrochem) | Not suitable | 0 years (first-stage prep only) | 15-25 years (zinc-rich + epoxy + PU 3-coat) |
| VOC content | Low-medium (50-150 g/L, mostly water-based) | 0 (mechanical) | High 250-450 g/L solvent-based (low-VOC < 250) |
| Environmental constraints | Confined spaces OK · no rain for 24 hr | Needs containment + extraction (heavy dust) · humidity < 85% | Humidity < 85% · surface > dew point + 3°C · temperature 10-35°C |
| Accepted in government TORs | ✗ Rarely (budget-limited maintenance only) | ✓ Standard — SRT / Department of Highways / EGAT / PEA | ✓ Standard — every C3-C5 steel-structure TOR |
| Suited to | Roof touch-ups / structures that cannot be taken out of service / tight budgets | New work + major re-coats + before high-spec coatings | Every job after sandblasting — the primary corrosion barrier |
| Saha delivers in 7-15 days | ORAFON Rust Stop 1L/5L/20L | Via SSPC-QP1 certified partner | ORAFON Zinc-Rich + Epoxy + PU finish 20L pail |
Specs per SSPC-SP1 to SP10, NACE No. 1-4, ISO 8501-1 (visual standards), ISO 12944-5 (paint systems), ASTM D520 (zinc dust), ASTM D4417 (profile measurement). Thai market average prices, May 2026 — actual project pricing depends on size + access + inspection/acceptance
Steel structure starting to rust?
Give us the scope (area in m² + environment class + the TOR cited) → our team promptly assesses the right method + total price for sandblast + ORAFON primer + top coat + lead time + ISO 12944 warranty certificate. Free site survey in Bangkok and the surrounding provinces
Related comparisons
Decide with confidence — compare from several angles
Read the related comparison tables — full specs, real prices, and a recommendation for each use.
FAQ
Common Questions
Q1.Can I just use rust converter and skip sandblasting?+
**Only for niche maintenance — not primary work**. Rust converter (tannic acid or phosphoric acid) chemically converts Fe₂O₃ (red rust) into stable iron tannate / iron phosphate, but:
• Only works on light rust (< 100 µm) where sandblast is impractical — rooftop tanks, undersides of operating bridges
• Loose scale must still be removed by hand/power tool (SSPC-SP3) — cannot apply over flaking rust
• Service life 3-7 years vs sandblast + epoxy 15-25 years
• Never apply epoxy primer over residual acid without top-sealing — unreacted acid degrades the bond
**Most Thai government TORs reject it** — Department of Highways, PEA, SRT specs require SSPC-SP10 (Near-White) minimum. Use rust converter only for budget maintenance or sandblast-inaccessible areas
Q2.SSPC SP6 vs SP10 vs SP5 vs ISO Sa 2 / 2.5 / 3 — what's the difference?+
Surface cleanliness levels after abrasive blasting — the two standards map to each other:
• **SSPC-SP5 / NACE No. 1 = ISO Sa 3 — White Metal** — removes 100% of rust, mill scale, and old paint (silver-white steel). Required for immersion, atomic energy, offshore C5-M
• **SSPC-SP10 / NACE No. 2 = ISO Sa 2.5 — Near-White Metal** — removes 95% (residual staining ≤ 5% area). Thai standard for C3-C4 industrial + petrochemical
• **SSPC-SP6 / NACE No. 3 = ISO Sa 2 — Commercial Blast** — removes 67% (residual ≤ 33%). For general epoxy systems in C2-C3
• **SSPC-SP7 / NACE No. 4 = ISO Sa 1 — Brush-Off Blast** — loose scale only (rarely specified)
**Anchor pattern (profile):** SP5/SP10 = 50-100 µm, SP6 = 35-75 µm. Measure with Testex tape (ASTM D4417 Method C) every 50 m² minimum
Q3.Does zinc-rich primer require a top coat?+
**Yes — always for outdoor service**. Zinc-rich epoxy (≥ 80% zinc dust in dry film, per ASTM D520 Type II + SSPC-Paint 20 Level 1) provides cathodic protection — zinc corrodes sacrificially before the steel. But:
• **Direct UV exposure** → zinc oxide bloom → bond failure → top coat peels
• Must be over-coated with epoxy intermediate (75-150 µm) + PU finish (50 µm) → standard 3-coat system
• Zinc-rich alone ฿180-280/m² (75 µm DFT); full 3-coat ฿380-580/m²
• Service life C5-M (offshore/petrochemical) 15-25 years vs non-zinc epoxy 8-12 years
**Exception:** indoor C2 dry — zinc-rich + sealer is sufficient (2-coat ฿250-350/m²). Thai Highway bridges, SRT rolling stock, EGAT transmission towers (dark green livery) all use 3-coat
Q4.Does Sahawatthanakit sell zinc-rich primers (e.g. ZINGA / Jotun / Hempel)?+
No — we do not sell any zinc-rich primer, so we have no manufacturer specs or prices to compare.
The anti-rust coatings we sell: **SGP-001** one-part PU, goes over surface rust without blasting, ฿8,100 per 1 US Gallon · **SGU-201** epoxy for wet or underwater steel, from ฿2,560 per Size S (A 1kg + B 0.2kg) (unit price, excluding 7% VAT) — see Industrial coatings (/en/paint)
Q5.Does Sahawatthanakit take on rust-protection, blasting or painting work?+
No — we sell materials only; we do not blast, paint or install.
• **Anodes** aluminium · zinc · magnesium · concrete — supply only, no installation (/en/rust)
• **Anti-rust coatings** SGP-001 goes over surface rust without blasting · SGU-201 for wet or underwater steel (/en/paint)
Tell us the job and the area on LINE OA @sahawatt1988 for a materials price
Free advice · a prompt, real quotation during business hours
Still not sure which one fits your job?
Tell us the job in a line — a Sahawatthanakit engineer checks the right rust-protection systems for you free, with a prompt quotation during business hours.
Used only to contact you · never shared with third parties