Short answer: National Block Filler is a ready-to-use, water-based polymer emulsion filler for sealing porosity and levelling concrete blocks, concrete and plaster, inside and outside. National’s method is one coat to seal the pores, 24 hours drying, then one coat for surface perfection, applied by roller, knife or blade. It dries matt white, contains antifungal and antibacterial additives and keeps for 18 months sealed.
Why fill blockwork
Open-textured blocks are full of pores. Painted directly, they absorb primer and emulsion unevenly and the texture shows through the finish. Block Filler closes the pores and levels the face so the paint system goes on evenly. Choose the right National filler for the job:
| Filler | Main job | Where | Tools | Recoat |
|---|---|---|---|---|
| Block Filler | Seal pores, level blocks, concrete, plaster | Interior and exterior | Roller, knife, blade | 24 h |
| Stucco Filler | Smooth interior walls before emulsion or enamel | Interior | See its data sheet | See its data sheet |
| Joint Filler | Fill gaps up to 1 cm | Interior and exterior | Scraper, knife, blade | Hard dry 24 h |
| Spray Plaster Fine | Smooth uneven or unplastered walls, 0.5 to 1 mm per coat | Interior | Spray-plaster gun or knife | 6 to 8 h |
Key facts
| Type | Water-based polymer emulsion filler |
| Surfaces | Concrete blocks, concrete and plaster; interior and exterior |
| Finish and colour | Matt, white |
| Volume solids | 48 ± 2% |
| Specific gravity | 1.60 |
| Coverage | Depends on the porosity of the surface |
| Drying at 30°C | Touch dry 1 to 2 hours; hard dry and recoat after 24 hours |
| Application | Roller, knife or blade |
| Thinning | Ready to use; clean tools with water |
| Shelf life | 18 months, sealed |
Step 1: prepare the blocks
National’s surface preparation guide for concrete, plaster and cement asks for a clean, dry, sound surface:
- Brush off loose material and dust with stiff brushes; pressure-wash exteriors (2200 psi minimum) and let them dry.
- Remove efflorescence (white salts) and any oil or grease.
- Remove laitance on cast concrete, which severely reduces adhesion.
- Fill wide joints or cracks first with Joint Filler.
Step 2: apply in two coats
- Seal coat: roll or knife one coat into the pores.
- Wait 24 hours at 30°C until hard dry.
- Perfection coat: a second coat to level the surface; sand lightly when dry if needed.
- Prime and paint with the system for your finish: the interior wall system inside, or Acrylic Rock or a texture outside.
How much to buy
National gives no fixed spreading rate because use depends on how porous the blocks are. The best way to estimate is a trial area: fill 1 to 2 m² with both coats, measure what you used, and scale up. Or send us your wall area and a photo of the blocks on WhatsApp.
Mistakes to avoid
- Painting the same day. The sheet gives 24 hours hard dry and 24 hours before the next coat.
- Thinning it. It is supplied ready to use.
- Filling over salts. Remove efflorescence first or it can push the coating off.
Frequently asked questions
Do I still need a primer after Block Filler?
Follow the primer specified by your finish’s data sheet, for example PVA Primer White or Acrylic Primer. The Block Filler sheet does not replace them.
Can it go outside?
Yes. National lists interior and exterior use.
Buy in Sharjah and Dubai
Al Tashteb Al Saree is an authorised National Paints retailer in Bu-Tina, Sharjah. Shop National Acrylic Primer 18 L, or message us on WhatsApp for Block Filler. See all National Paints products.
Sources
All via AEC Online, retrieved 4 November 2026. The main image is illustrative.
