Short answer: National M.I.O. Undercoat is an alkyd-based undercoat pigmented with micaceous iron oxide, for external structural steel, cranes, towers, bridges and pipelines in mild industrial environments. National describes good water resistance and a barrier against water vapour. Apply one 40-micron coat over Zinc Phosphate Primer; it covers about 12 m² per litre and needs 14 hours before the topcoat at 30°C. It comes in light grey only.
Why an MIO undercoat for external steel
Micaceous iron oxide is a flaky, plate-like pigment. National’s data sheet credits this undercoat with good water resistance and a barrier against water vapour, which is why it specifies it, rather than its general Synthetic Undercoat, for exposed steel structures. It sits in a three-product system with Zinc Phosphate Primer below and Enamel Gloss II M.I.O. on top.
| M.I.O. Undercoat | Synthetic Undercoat | |
|---|---|---|
| Listed for | External structural steel, cranes, towers, bridges, pipelines | All interior and exterior paintwork |
| Pigment | Micaceous iron oxide | Standard |
| Colour | Light grey only | White, grey |
| Volume solids | 48 ± 2% | 52 ± 2% |
| m² per litre | 12 (at 40 µm) | 13 to 17 |
| Recoat at 30°C | 14 h | 12 h |
Key facts
| Type | Alkyd undercoat with micaceous iron oxide, single pack |
| Exposure | External, mild industrial |
| Finish and colour | Matt, light grey |
| Volume solids | 48 ± 2% |
| Specific gravity | 1.55 ± 0.05 |
| Spreading rate | 12 m² per litre (theoretical) |
| Film per coat | 40 microns dry (85 wet) |
| Drying at 30°C | Touch dry 3 hours; recoat 14 hours; hard dry 24 hours |
| Application | Airless spray (2200 psi, tip 0.015 to 0.017 inch), air spray (50 psi) or brush |
| Thinning | Airless 5 to 10%, air 10 to 15%, brush 10%, with National G.P. Thinner |
| Flash point | 38°C |
| Shelf life | 18 months, sealed |
Step 1: prepare and prime
- Degrease, then blast the steel to Sa 2.5 (or clean to St 2 for lighter work), following National’s surface preparation guide.
- Apply Zinc Phosphate Primer, one coat at 40 microns, and wait 10 hours.
Step 2: complete the system
| Coat | Product | Dry film | Wait before next coat (30°C) |
|---|---|---|---|
| 1 | Zinc Phosphate Primer | 40 µm | 10 h |
| 2 | M.I.O. Undercoat | 40 µm | 14 h |
| 3 and 4 | Enamel Gloss II M.I.O. (or Gloss I / Gloss II) | 2 × 35 µm | 8 h between coats (Gloss II MIO) |
| Total | 150 µm | ||
What the numbers mean
Volume solids × 10 ÷ dry film gives coverage: 480 ÷ 40 = 12 m² per litre, the sheet’s figure. Wet film = 40 ÷ 0.48 ≈ 83 microns, close to the 85 recommended.
How much to buy
| Steel area | M.I.O. Undercoat, 1 coat |
|---|---|
| 100 m² | about 8.3 L |
| 250 m² | about 21 L |
| 500 m² | about 42 L |
| 1,000 m² | about 83 L |
Mistakes to avoid
- Topcoating at 12 hours out of habit. This undercoat needs 14 hours at 30°C, longer than Synthetic Undercoat.
- Using it as the finish. It is an undercoat; National’s system always tops it with two enamel coats.
- Expecting it to cover heavy-duty marine or chemical exposure. National lists it for mild industrial environments; its specifier uses epoxy systems for heavy duty (see steel painting systems).
Frequently asked questions
Can I use it over Red Oxide Primer?
National’s data sheet specifies Zinc Phosphate Primer underneath. Ask us before changing the system.
Does it come in other colours?
No, the sheet lists light grey only. The colour comes from the topcoat.
Buy in Sharjah and Dubai
Al Tashteb Al Saree is an authorised National Paints retailer in Bu-Tina, Sharjah. Message us on WhatsApp for the full MIO system and G.P. Thinner.
Sources
- National M.I.O. Undercoat data sheet (PDF)
- National Zinc Phosphate Primer data sheet (PDF)
- National Enamel Gloss II M.I.O. data sheet (PDF)
- National Paints surface preparation guide, Sec. A-2 (PDF)
All via AEC Online, retrieved 4 November 2026. Quantities and timings are our own calculations. The main image is illustrative.
