Film thickness and spreading rate
Wet film thickness needed for a target dry film (with thinning)
- where
- volume solids after thinning (%)
- volume solids as supplied (%)
- thinner added, as a percentage of the paint volume (%)
- wet film thickness to apply (µm)
- target dry film thickness per coat (µm)
Explain this equation
Works out the wet film you need to put on, checked with a comb gauge, to reach a specified dry film thickness, allowing for any thinner you have added.
VS is the volume solids from the data sheet. Adding thinner th per cent spreads the same solids through more liquid, so VSt, the solids in the thinned mix, is lower. DFT is the dry film you are aiming for and WFT is the wet film that will give it.
The first line scales the solids down by the extra volume: 100 parts of paint become 100 + th parts of mix. The second line runs the dry film equation backwards, dividing the target dry film by the solids fraction.
Sense check: Hempel's example of 56% solids thinned 15% gives about 48.7% solids, so a 75 µm dry film needs about 154 µm wet. With no thinner, VSt equals VS.
Watch out: round the answer up to the next comb gauge step, and never thin beyond the data sheet maximum.
Works out the wet film you must lay down, checked with a comb gauge, to reach the dry film thickness in the specification.
Enter your values to see the result.
Good to know
This is the method in Hempel's data sheet notes: wet film = dry film × 100 / volume solids, with the volume solids first reduced for any thinner added. Round up to the next step on your comb gauge. A few per cent of thinner makes little difference; 10–20% (for example 1 part water to 10 or to 5 parts paint, the limits on many Dulux Trade data sheets) makes a measurable one.
Common mistake
Thinning for spraying and then applying the same wet film as before. The dry film comes out thinner, so coverage and opacity suffer and an extra coat may be needed.
Limits
Assumes the thinner is fully volatile and volumes simply add. For porous substrates allow for absorption; the data sheet usually gives a separate, thinned mist or sealer coat for these.
Sources
- Hempel technical guideline: Mixing and thinning (volume solids after thinning = VS × 100 / (100 + % thinner); worked example 56% VS, 15% thinner, 75 µm DFT)
- Hempel: Explanatory notes to product data sheets (theoretical spreading rate = volume solids % × 10 / DFT µm; WFT = DFT × 100 / volume solids; volume solids to ISO 3233-1; drying and overcoating times depend on temperature)
- Dulux Trade Weathershield Smooth Masonry Paint data sheet (volume solids 38% white; do not apply below 7 °C or above 80% RH; recoat 2–4 h; VOC max 10 g/L against 40 g/L limit, cat. A/c; airless thinning up to 1 part water to 5 parts paint)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.