National Synthetic Undercoat: Alkyd Undercoat Under Enamel Paint

Short answer: National Synthetic Undercoat is an alkyd-based, non-absorbent intermediate coat with good opacity and enamel holdout. It goes between the primer and the enamel (or emulsion) on steel, wood and walls, inside and out. Apply one coat at 30 to 40 microns dry; it covers about 13 to 17 m² per litre and needs 12 hours before the finish coat at 30°C. It comes in white and grey, with other colours on request.

What an undercoat does

National describes Synthetic Undercoat as non-absorbent with strong enamel holdout: it stops the primer or filler drawing in the gloss coat, so the enamel stays even and glossy, and its opacity hides the primer colour. That is why almost every National alkyd enamel system includes it. Choose a different intermediate coat for external steel structures:

Undercoat	National lists it for	Colours	m² per litre	Recoat at 30°C
Synthetic Undercoat	All interior and exterior paintwork	White, grey	13 to 17	12 h
M.I.O. Undercoat	External structural steel, cranes, towers, bridges, pipelines	Light grey	12	14 h

Key facts

	Decorative sheet	Protective sheet
Type	Alkyd undercoat, single pack, matt
Volume solids	52 ± 2%
Specific gravity	1.50 ± 0.05	1.55 ± 0.05
Spreading rate	13 to 17 m² per litre
Film per coat	30 to 40 µm dry (60 to 75 wet)
Drying at 30°C	Touch 2 to 3 h; recoat 12 h; hard 24 h	Touch 4 h; recoat 12 h; hard 24 h
Application	Brush, roller or spray (airless 2200 psi, tip 0.015 to 0.020 inch; air 50 psi)
Thinning	5 to 15%	Airless 5 to 10%, air 10 to 15%, brush 10%
Thinner	National G.P. Thinner
Flash point	38°C
Shelf life	18 months, sealed

Every National system that uses it

Surface	Primer and preparation	Undercoat	Finish
Steel	Red Oxide, Red Lead or Zinc Phosphate × 1	1 coat	Gloss I or Gloss II × 1 to 2
Galvanised, aluminium	Wash Primer, then Zinc Chromate	1 × 40 to 50 µm	Gloss I or II × 1
Wood (doors, frames)	Wood Primer × 1, Stucco Filler × 2	1 coat	Gloss I or II × 1
Concrete and walls	PVA Primer Sealer × 1, Stucco Filler × 2	1 coat	Gloss I, Flat/Matt Enamel or Eco Enamel × 1

Heavier steel builds in the specifier

Exposure	Red Oxide	Synthetic Undercoat	Gloss I or II	Total
Non-polluted air (BS 5493 Table 4F, SF 7)	2 × 35 µm	2 × 35 to 40 µm	1 × 25 to 40 µm	165 to 190 µm
Polluted air (BS 5493 Table 4F, SF 8)	2 × 35 µm	2 × 35 to 40 µm	2 × 25 to 40 µm	190 to 230 µm

Step by step

1. Let the primer or filler dry fully, following its own data sheet.
2. Stir, and thin with G.P. Thinner (about 10% for brush, per the protective sheet).
3. Apply 30 to 40 microns dry; aim for about 60 to 75 microns wet.
4. Wait 12 hours at 30°C, lightly de-nib if needed, then apply the enamel.

Use grey undercoat under darker enamels and white under pale ones to help the finish cover. This is general painting practice, not a data sheet rule.

How much to buy

Area	1 coat	2 coats (BS 5493 builds)
20 m² (about four doors, both sides)	1.2 to 1.5 L	2.4 to 3.1 L
50 m²	2.9 to 3.8 L	5.9 to 7.7 L
100 m²	5.9 to 7.7 L	11.8 to 15.4 L

Frequently asked questions

Can I skip the undercoat?

National’s enamel systems include it. The exceptions are its industrial products: Industrial Quick Drying Enamel and Camouflage Enamel go straight over Red Oxide Primer.

Can emulsion go over it?

The protective sheet says it can be overcoated with National alkyd finishes or National emulsion paints.

Buy in Sharjah and Dubai

Al Tashteb Al Saree is an authorised National Paints retailer in Bu-Tina, Sharjah. Message us on WhatsApp for Synthetic Undercoat in white or grey, plus G.P. Thinner.

Sources

• National Synthetic Undercoat, decorative data sheet (PDF)
• National Synthetic Undercoat, protective data sheet (PDF)
• National Paints systems specifier (PDF)

All via AEC Online, retrieved 4 November 2026. Quantities are our own calculations. The main image is illustrative.