Why the same 200 µm specification can mean different paint consumption on two products, and how volume solids drives your material estimate and cost per square metre.
Two products both specified at 200 µm dry film thickness can consume very different amounts of paint. The reason is volume solids — the percentage of a liquid paint that remains as dry film after the solvent has gone.
The arithmetic
Dry film thickness equals wet film thickness multiplied by the volume solids fraction. To apply 200 µm dry with a coating at 60% volume solids, you must spray roughly 333 µm wet. The same 200 µm with a 75% solids product needs only about 267 µm wet. That is a 20% difference in material consumed for the same protected surface.
For theoretical consumption per square metre, divide the dry film thickness in microns by ten times the volume solids, expressed in litres. In practice add a loss factor for overspray, dead space and surface profile, commonly ten to thirty per cent on structural steel and considerably more on complex lattice work.
Why low-solids products cost more than they look
A cheap paint with 45% volume solids needs roughly 444 µm wet to reach the same 200 µm dry, and it must be sprayed in more passes with longer flash-off between them. The extra labour, extra thinner and the greater risk of runs and sags usually outweigh the price advantage.
What to check on the data sheet
- Volume solids, not weight solids. Weight solids figures are higher and are sometimes quoted to flatter a product.
- The stated test method and any tolerance. Volume solids are normally quoted for the mixed product at a defined temperature, and can vary by a couple of percentage points between batches.
- Recommended wet film thickness or spreading rate, which is the manufacturer's own conversion of the dry film requirement.
- Whether the figure applies to the mixed or the base component. Mixing ratio errors throw the calculation off immediately.
Practical consequences
When comparing quotations for the same project, do not compare price per litre. Compare litres required for the specified dry film thickness over the actual surface area, then add the loss factor. Only that number, multiplied by the unit price and added to labour, gives a comparable cost per square metre.
For high-build and high-solids products, the same logic explains why they can reach a thick film in fewer passes. On a large structural steel project the reduction in labour and overspray is often worth more than the small premium per litre.