When calculating the quantity of paint required for a project, many factors need to be considered:
- Surface area to be painted
- Specified dry film thickness (in mils or microns)
- Conversion factors (imperial to metric etc.)
- Non-volatile content of the paint at application viscosity
- Wastage factor (very low for brush/roller application, very high for spray application on complicated structural members)
calculating the surface area to be painted can be a major nightmare, especially if you struggled in Algebra class, and architects these days are competing to create weirder and weirder structures just to complicate things further.
Specified dry film thickness is crucial to the longevity of a painted steel structure, and may be specified in either mils or microns. Each layer of a paint system performs a different role, so the specified thickness for each layer may be different.
Sometimes it may be necessary to convert a quantity from imperial units into metric units or vice versa. This can get especially confusing when converting coverage rates in sq. ft/gal to sq. m/L, due to the double conversion.
The non-volatile content of a coating at application viscosity is expressed as a percentage by volume. Thus a coat of 50% solids paint applied at a wet film thickness of 100 microns will shrink back to a dry film thickness of 50 microns as the solvents evaporate from the matrix. Often a Technical Data Sheet for a coating will give the theoretical coverage rate for a nominated dry film thickness.
The wastage factor is dependent on application type as well as complexity of the shape to be coated. When applying the paint by brush or roller this will be almost zero. When applying the paint by spray however, there will always be a certain amount of overspray. The estimator must rely on instinct a little here, because there is no hard-and-fast way of calculating wastage. As a general rule of thumb, allow 20% loss for simple structures and up to 60% loss for complex structures.
