Short answer: a paint data sheet is a one-page spec. Five numbers matter most: volume solids (how much of the liquid becomes dry film), spreading rate (square metres per litre), dry and wet film thickness (how thick each coat should be), drying and recoat times and the thinning limit. Once you can read these, you can check any quote, calculate litres yourself and spot a coat that was applied too thin.
Why the data sheet beats the label
The tin label gives a name and a rough coverage. The technical data sheet gives the figures the manufacturer tested, in the conditions it tested them. National Paints publishes one for each product, and every line has a job. We use the National Matt Emulsion sheet below because its numbers are easy to follow, and the same logic applies to any National product.
The numbers that matter, one by one
| Line on the sheet | Matt Emulsion value | What it tells you |
|---|---|---|
| Volume solids | 42 ± 2% | About 42 parts in 100 of the wet paint stay on the wall as dry film; the rest evaporates |
| Spreading rate | 8.5 to 10.5 m² per litre (theoretical) | Area one litre covers at the stated film thickness |
| Film per coat | 40 to 50 microns dry (95 to 120 wet) | The thickness you are aiming for in each coat |
| Drying at 30°C | Touch dry 1 hour; recoat 2 to 4 hours | How long to wait before the next coat |
| Thinning | 5 to 10% with water | The most water you should add |
| Shelf life | 18 months, sealed | How long an unopened tin keeps |
Volume solids: what is left after drying
Paint is part solids (binder and pigment) and part liquid that evaporates. Volume solids tells you the share that stays. A paint with higher volume solids builds more film from each litre, which is why two paints of the same price per litre can cost different amounts per covered square metre.
Spreading rate: the theory and the wall
The spreading rate on the sheet is theoretical. It assumes a smooth, non-absorbent surface and no waste. Rough plaster, open blockwork and spray losses all lower the real figure, so treat the sheet number as the best case and round your order up. A suitable primer or filler reduces how much the wall soaks up, which is why National’s wall systems start with one. See our interior wall system guide.
Wet and dry film: how the sheet figures connect
These numbers are linked by simple arithmetic, which is a good way to check that you are reading the sheet correctly:
- Coverage = volume solids × 10 ÷ dry film. For Matt Emulsion: 420 ÷ 40 = 10.5 and 420 ÷ 50 = 8.4 m² per litre, which matches the sheet’s 8.5 to 10.5.
- Wet film = dry film ÷ volume solids. A 45-micron dry coat needs about 107 microns wet (45 ÷ 0.42). The sheet’s 95 to 120 microns wet for 40 to 50 dry follows the same rule.
- Thinning adds liquid, not solids. Adding 10% water lowers the solids in the tin to roughly 38%, so a wet coat of the same thickness leaves less dry film. Stay within the sheet’s 5 to 10% and do not use thinning to stretch the paint.
Worked example: litres for a bedroom
A bedroom of 4 × 4 m with 3 m walls has about 45 m² of wall once door and window openings are taken out. At the sheet’s 8.5 to 10.5 m² per litre, one coat needs 4.3 to 5.3 litres, so two coats need about 8.6 to 10.6 litres of Matt Emulsion. Round up to a full pack, then add a little for touch-ups. These are our own calculations from the sheet’s theoretical rate; measure your own walls before ordering.
Reading drying and recoat times
Drying times on a data sheet are quoted at a stated temperature, here 30°C. Touch dry means the film will not mark when lightly touched. Recoat is the window in which the next coat can go on. Humid conditions slow drying, so treat the shortest time as a minimum rather than a target, and do not recoat a coat that still feels cool or soft.
Common mistakes when reading a sheet
- Treating the spreading rate as a guarantee. It is the theoretical best case on a smooth surface.
- Comparing paints by price per litre. Compare cost per square metre at the same dry film instead.
- Skipping the system. The sheet lists the primer and number of coats the figures assume. Leave one out and the numbers no longer apply.
- Ignoring the surface. National’s surface preparation guide asks for clean, dry, sound surfaces before any coat.
Keep going
Our National Matt Emulsion guide applies these numbers to the full system, and How Much Paint Do You Need? works through rooms of different sizes. Not sure what a line on a sheet means? Al Tashteb Al Saree is an authorised National Paints retailer in Bu-Tina, Sharjah. Message us on WhatsApp with the product and your wall area.
Sources
- National Matt Emulsion technical data sheet (PDF)
- National Paints surface preparation guide, Sec. A-2 (PDF)
Data sheet figures via AEC Online, retrieved 5 November 2026. Calculations are our own. The images are illustrative.
