A Critical Element of Surface Preparation Before Coating
Understanding surface profile - In the blast cleaning process, grains of abrasive are propelled with great force and energy at the work surface. Upon impact, the grains 'dig' into and then rebound out off the surface leaving a rugged, miniature 'mountain-and-valley' finish.
This surface roughness/etch/texture is the surface profile.
Surface profile is critical to coating performance by
1) increasing the surface area
2) providing a 'key/tooth/anchor pattern' for the coating to lock and adhere to.
The difference between Surface Profile and Class of Blast
Surface profile is concerned with the 'shape' of the surface finish (and measuring the size of the 'shape' created) whereas Class of Blast is concerned with 'cleanness' of the surface finish. (putting it another way - Class of Blast is determining to what degree the rust, paint and other contaminants have been removed).
Both the Profile and the Class of Blast are important features of the surface finish and need to be separately specified in preparing a blast cleaned steel surface.
The Pitfalls of Surface Profile
Excess Profile - While an absence of profile can be detrimental to coating adhesion, it can be equally disastrous to have an excessive profile height causing premature rusting and coating failure. In addition more profile means using more paint to cover the surface! Consider these cases...
Rule of thumb #1: Profile height should not exceed the primer coat DFT.
Rule of thumb #2: Profile height should not exceed 1/3 the total coating system DFT.
Embedment - Embedment of abrasive particles in the surface is a threat posed by fissile, irregular shaped abrasives. The embedded particle or fragment can stand out as a 'rogue' peak above the surrounding profile and protrude through the applied coating.