What the figure means
A liquid coating is a blend of binder, pigments, additives and, in most cases, solvent or water. Only the non-volatile part stays on the steel or concrete. Volume solids expresses that part as a share of the liquid volume. A product with low volume solids loses a large fraction of its thickness as it dries; a product close to full solids loses very little.
Volume solids is not the same as weight solids. Pigments are dense, so a coating can show a high solids content by weight and a lower one by volume. Film thickness and coverage depend on volume, which is why the volume figure is the one used on site.
The film thickness relationship
The core relationship is simple:
- dry film thickness equals wet film thickness multiplied by volume solids, divided by 100;
- wet film thickness to apply equals the target dry film thickness multiplied by 100, divided by volume solids.
If the coating is thinned, the added thinner increases the liquid volume without adding solids, so the wet film must be thicker to reach the same dry film thickness. The data sheet gives the corrected values or the permitted thinning range.
Volume solids and coverage
| Quantity | What it depends on | Where to find it |
|---|---|---|
| Theoretical spreading rate | Volume solids and target dry film thickness | Technical data sheet |
| Practical spreading rate | Theoretical rate less application losses | Estimated from method, shape and conditions |
| Wet film to apply | Target dry film and volume solids, adjusted for thinning | Calculated, then checked with a wet film gauge |
Losses come from overspray, wind, complex shapes, surface profile and material left in hoses and pails. They vary with the job and are not part of the volume solids figure.
Test methods and limits
ISO 3233 and ASTM D2697 determine volume solids under laboratory conditions. Results depend on the cure conditions and on the film tested, so two products are best compared on values obtained by the same method. Field checks then confirm the result: a wet film thickness gauge during application and dry film measurement per SSPC-PA 2 or ISO 19840 after cure.
Our protective coatings service bases film build and material quantities on the data sheet values, not on assumed coverage.