Concept · Conditions and application

Volume solids, wet film and dry film

Volume solids links the wet film you apply to the dry film you get. The formula, the thinner correction and the spreading rate.

Volume solids (VS) is the share of a paint's volume that stays on the surface as dry film. The rest is solvent that evaporates. The data sheet gives it as a percentage. To convert a thickness quickly, use the WFT to DFT calculator guide.

What happens in the film

Liquid paint holds binder, pigment and additives in a solvent. After application the solvent leaves the film. The film shrinks until only the solids remain. The wet film is therefore always thicker than the dry film. The two are equal only at 100 % VS.

The same wet film gives different dry films with different paints. A 250 µm wet film leaves 150 µm at 60 % VS and 200 µm at 80 % VS. The inspector must therefore know the VS of the product in the can.

Volume solids is a share of volume. Do not confuse it with solids by mass. A heavy pigment such as zinc dust makes the share by mass much higher than the share by volume.

Thinner is the second reason why the wet film must be thicker than the dry film. It adds volume but no solids. If you add 10 litres of thinner to 100 litres of paint, the same solids now spread through 110 litres. The paint as applied has a lower VS, so the applicator must put on a thicker wet film for the same dry film.

The formulas

DFT = WFT × VS ÷ 100, so WFT = DFT × 100 ÷ VS.

With thinner: WFT = DFT × (100 + thinner %) ÷ VS. Thinner % is the volume of thinner for each 100 volumes of paint.

Theoretical spreading rate in m²/L = 10 × VS ÷ DFT in µm. The 10 has a simple reason. One litre of dry film at 100 % VS covers 1,000 m² at 1 µm. So 1,000 ÷ DFT gives the cover at 100 % VS, and VS ÷ 100 scales it.

Practical spreading rate = theoretical rate × loss factor. The loss factor is below 1 because overspray, wastage and paint left in the pot lower the cover.

In the field

1. Read the data sheet. Note the VS, the DFT range, the thinner type and the thinner limit.
2. Calculate the target WFT from the specified DFT, the VS and the thinner actually added.
3. Measure the WFT right after application, while the coat can still be corrected. ISO 2808 lists the comb, the wheel and the dial gauge (ISO 2808). The comb is the most common in the field.
4. Treat a comb reading as an approximate control. ASTM D4414 calls notch gauges neither accurate nor sensitive (ASTM D4414).
5. Tell the applicator to adjust the film while the paint is wet.
6. After cure, measure the DFT with a gauge (DFT measurement). The cured film decides acceptance, not the wet film.

A worked example

The specification asks for 200 µm DFT. The paint has 64 % VS and the crew adds 5 % thinner. The target wet film is 200 × 105 ÷ 64 = 328 µm.

Check: 328 × 64 ÷ 105 = 200 µm.

The theoretical spreading rate is 10 × 64 ÷ 200 = 3.2 m²/L. The job is 500 m², with 30 % losses, so the loss factor is 0.70. The practical rate is 3.2 × 0.70 = 2.24 m²/L. The paint order is 500 ÷ 2.24 = about 223 L, rounded up to whole packs.

Check: 500 ÷ 3.2 = 156.25 L with no losses, and 156.25 ÷ 0.70 = 223 L. The 5 % thinner is extra volume. It does not change the litres of paint to order.

The WFT to DFT calculator and the paint consumption calculator do the same arithmetic.

Wet film control matters because a thin coat is easy to fix in the first minutes. After cure, a repair costs extra labour. Repair can also bring dirt and poor adhesion between the coats.

What the inspector records

• The product, the batch and the VS from the data sheet.
• The thinner type and the percentage added.
• The target WFT, each WFT reading, the time and the place.
• The gauge used for the wet film, and later the DFT results.

Exam traps

• Trap: calculating the WFT as DFT × VS ÷ 100. The wet film is never thinner than the dry film.
• Trap: leaving the thinner out of the WFT target.
• Trap: adding the thinner to the paint order. Thinner changes the wet film, not the litres of paint.
• Trap: using VS as 60 in one place and 0.60 in another. Keep one form in a line.
• Trap: multiplying the theoretical rate by a consumption factor. Multiply it by the loss factor, which is below 1.
• Trap: accepting a coat on the wet film reading. The DFT on the cured film decides.

How the exams treat it

The CIP 1 practical has a wet film gauge station worth 5 of 100 points. The FROSIO book includes calculations. The guide WFT to DFT guide has more worked examples, and the consumption guide goes on to litres to order.

Use it: WFT ↔ DFT calculator · WFT ↔ DFT guide

90 practice items in the members area mention this topic (CIP 1: 34 · CIP 2: 20 · FROSIO written: 36).

Related

• Standards: ISO 2808 · ASTM D4414
• Defects: Mud cracking · Runs and sags · Wrinkling · Overspray · Brush marks
• Conditions and application: Dew point and the 3 °C margin · Pot life and induction time · Drying, curing and recoat windows · Stripe coats