Due to extra surface area created by the surface profile and roughness with required peak coverage, extra paint is needed. A rule of thumb for the determination of the approximate extra paint to fill a SAND, GRIT or Bristle Blast profile is to multiply the peak to valley height (profile) times 0.5 and add this to the Dry Film Thickness you are trying to achieve.
For a peak-to-valley height of 10 mils, an additional quantity of paint equal to a full coat at 5 mils dry film thickness will be required. Because a SHOT Blast Profile is smoother, the amount of paint would be slightly less, and you might want to use 0.25 times the blast profile to calculate the additional amount of paint.Example calculation of extra paint
The rule for paint is 1 gallon of paint applied at 1 mil will cover 1,604 sq ft. wet film thickness on a smooth surface. 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. Given a series of peak to valley heights of a blast cleaned surface, the greater the peak to valley height, the more paint will be required to fill the profile before a measurable thickness of paint is applied.
In the above drawings, if Figure #1 represents a 1,604 sq ft area and we needed a 2-mil coating of 100% solids to paint, we would need to apply 2 gallons of paint. It should be easy to determine that Figure #2 has a greater surface area than Figure #1. If Figure #2 is the same area with a Sand or Grit blast profile, you would APPROXIMATE 3 gallons of paint to get 2 mil Dry Film Thickness (DFT).
Understanding the Effect of Profile Effect on DFT Measurements?
If you calculate the Wet Film Thickness required to get a dry film thickness and you don’t account for the profile, you will be off. When you measure a Wet Film Thickness you are measuring the amount of paint ABOVE the peaks.
When using a DFT Gage the meter must establish a zero point. When you have a surface with peaks and valleys, there is no clear line where the “Zero Point” is. There are areas in the profile that are LESS THAN THE ZERO POINT. The meter will not register any paint as being applied to the surface until it is greater than the zero point.
In the above drawing, if the DFT gage perceives the “Zero Point” to be above the orange paint, any paint below the dashed line will not be measured. This is explained in greater detail in the section on Dry Film Thickness in the Coating Inspection Handbook.
Consistency for a surface profile is key for the proper math for the proper coverage to meet the specification. Standards are also available from several organizations that provide direction for using various methods to obtain an anchor profile measurement.
This article was written by Tom Swan from MTEST.