Products and spraying
Spray paint quantity allowing for transfer efficiency (overspray)
- where
- paint to buy for spraying (L)
- paint needed on the surface (L)
- transfer efficiency, share reaching the surface (%)
- paint left in pump and hose (L)
- paint lost to overspray and equipment (L)
- extra paint compared with the surface quantity (%)
Explain this equation
Scales up a paint order for airless or HVLP spraying, because some of the paint leaving the gun never lands on the surface.
Qn is the paint the surface itself needs, as you would work out for rolling. TE is the transfer efficiency: the percentage of sprayed paint that ends up on the work. Vh is paint left in the pump and hose that you will not get back. Qs is what to buy, W the waste and E the extra as a percentage.
If only TE per cent of what you spray arrives, you have to spray 100 / TE times the surface quantity. The hose and pump content is a fixed amount added on top.
Sense check: at 80% transfer efficiency, 20 litres on the wall needs 25 litres sprayed, 25% extra, in line with the 15–25% more than rolling decorators report. Open steelwork or railings can drop to 50% or less, doubling the paint.
Watch out: wind, distance, pressure and technique all change transfer efficiency; check consumption on the first area.
Scales up the paint quantity for spraying, because only part of what leaves the gun ends up on the surface.
Enter your values to see the result.
Good to know
Transfer efficiency is the fraction of sprayed paint that lands on the work. On small parts it is low (published US Navy data: conventional air 15–30%, airless 20–40%, HVLP 50–65%). On large flat walls and ceilings airless does much better: decorators report about 15–25% more paint than rolling, which corresponds to a transfer efficiency of roughly 80–87%. The 80% default is at the cautious end of that; set your own figure from past jobs. Add the paint that stays in the pump and hose if you will not recover it.
Common mistake
Ordering the rolled quantity for an airless job and running short, or using a wall figure for spraying railings, spindles or open steelwork where most of the paint misses.
Limits
Transfer efficiency depends on pressure, tip size, gun distance, technique, wind and the shape of the work. Treat the result as an estimate and check consumption on the first area.
Sources
- Naval Facilities Engineering Service Center: High transfer efficiency paint spray systems, P2 opportunity data sheet (typical transfer efficiency on parts: conventional air 15–30%, airless 20–40%, air-assisted airless 25–40%, HVLP 50–65%)
- Squote: Spraying vs rolling (airless spraying on walls uses approximately 15–25% more paint per m² than rolling)
- 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)
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.