Theoretical spreading rates only provide an approximate guide to the volume of paint required to cover a known area at a given thickness.
There are several other key factors to consider which play a significant part in converting theoretical spreading rate figures to more realistic practical spreading rates.
Practical paint coverage is affected by:
Blast Profile
The effect of blast profile is important in calculating estimates of paint quantities required especially in cases in which the specification requires application of a minimum dry film thickness.
For a series of peak to valley heights of abrasive blast cleaned surfaces the greater the peak to valley height the more paint will be required to fill the profile before application of measurable thickness commences.
We recommend use of a simple method for determination of the approximate extra paint quantity required to fill the blast profile. This involves calculation of a figure equal to half the peak to valley height and assumes a requirement for extra paint to be applied equal to this thickness.
For example, for a peak to valley height of 50 microns, an additional quantity of paint equal to a full coat at 25 microns dry film thickness will be required.
Similarly, for a peak to valley height of 100 microns, an additional coat equal to 50 microns dry film thickness will be required, and so on. The influence of blast profile is particularly important in calculating quantity of primer required in which a minimum dry film thickness is specified.
The influence of blast profile is less severe on surfaces which have been shotblasted compared with those which have been gritblasted due to the smoother rounded nature of the profile. Additional quantities of paint required on shotblasted surfaces would therefore be expected to be less than the figures quoted above.
This concept should not be applied to the case of prefabrication or shop primers. These products are formulated at low volume solids with relatively quick drying solvents designed to apply low dry film thicknesses up to 25 microns which more accurately follow the profile than high build anticorrosive primers.
Over-Application or Uneven Application of Paint
Additional quantities of paint which may be included under the general heading of losses, can occur when sprayers do not pay due care and attention to their work. This may lead to significant extra quantities of paint being applied at an unnecessary extra cost.
In order to avoid this type of ‘loss’ it is important that sprayers are trained in methods of carefully controlling and monitoring application of products to achieve the correct target thickness.
The use of controlled fluid pressures and monitoring of applied wet film thickness using appropriate gauges is important.
Wastage
This is generally determined by the shape and complexity of items to be painted. The smaller the cross-section of the item or the more complex the shape, the greater the wastage factor due to overspray.
Choice of correct tip aperture and angle plus use of minimum tip pressure to assist in reducing the effects of overspray wastage are important.
Weather Conditions
Losses due to weather conditions are more important to site painting operations than to inhouse painting. The effects of wind speed can have a dramatic effect upon losses due to the effects of paint particles being carried away from the surfaces being painted.
Losses in Empty Containers and Equipment
There will always be a small quantity of paint retained in used containers and in equipment including airless spray pumps and fluid hoses at the conclusion of work. This type of wastage may be minimised by the adoption of good housekeeping practices. For instance by scraping inside containers to transfer the maximum possible quantity.
In the case of 2-pack materials it is important to mix as little as is required to keep ahead of the spraying operation to avoid unnecessary wastage.
Guidance for Calculation of Practical Spreading Rates
Experienced estimators and quantity surveyors using products they are familiar with are able to accurately predict estimates of paint required for a particular job on the basis of their knowledge. Wastage and losses can vary significantly from one job to another.
All factors considered it should be possible to keep total losses down to around 30% on heavy steel plate. In the case of lighter structural steel items typical loss factors may increase to as high as 65% or even 80% in extreme situations. The method of calculation involves a reduction in the theoretical spreading rate to arrive at a practical figure.
For a product giving a spreading rate of 5 square metres per litre and applying a 30% loss factor, the practical coverage rate will equate to:
5 x 0.7 = 3.5 square metres per litre
For the same product and applying an 80% wastage factor the practical coverage rate will become:
5 x 0.2 = 1.0 square metres per litre
The calculation should always be based upon an appropriate reduction of the spreading rate rather than an increase in the volume of paint calculated for a particular wastage factor.
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