Surface cleanliness, -profile, -roughness
A clean and well prepared surface is crucial for a successful coating performance.
The pre-treatment process of an industrial coating job often consumes more time and budget than the actual paint application itself.
There are various methods and systems to prepare a surface prior to the coating application. Depending on the project and job specifications one could choose for hand cleaning like brushing, grinding or using needle hammers. Often grit blasting is utilised or shot blasting in automatic line for example. If no anchor pattern is required or already in place (ultra) high pressure water jetting, chemical cleaning or a simple wash down might be the solution.
Although it seems quite obvious often the parameters cleanliness and profile are mixed up.
Cleanliness is providing information about the level of contaminants which are left on the surface. This contamination could be (blast) dust, soluble salts like chlorides, old paint residues, oil and fat, (flash) rust, mil scale, condensation and moisture, amines, etc..
Surface contamination can be caused in many different ways like insufficient cleaning, residue left after (acid) rain showers, sea salt left on the surface in maritime environments, soluble salts introduced by contaminated recycled blasting media and many others.
Therefore it is important to check for surface cleanliness at the right moment in the process. It is necessary to check the levels of cleanliness after the surface prep but keep in mind during the “open time” (which varies from hours to days) of the steel new contaminations can be introduced when the work is insufficiently shielded.
Golden rule: Always inspect for surface cleanliness just prior to paint application!
Surface profile, also addressed as surface roughness or anchor pattern is another parameter that plays an important role in the surface prep process. Besides a clean surface a certain roughness may be required to assure proper adhesion of the paint or coating to the surface.
By creating a rough surface with “peaks and valleys” one is more or less increasing the total surface area thus creating a larger bonding area for the paint to stick. Paint specifications often require a certain surface roughness or anchor profile in order to guarantee the performance of coating system. A surface profile which is too low may cause adhesion problems. However a surface profile which is too coarse could effect that the primer coating is not covering the “peaks” of the anchor pattern which may lead to premature corrosion or other problems with the intermediate or top coat. A coarse surface also leads to an increase of the paint consumption.
Creating a specified surface profile is not an easy job. The profile is determined by the size, type and hardiness of the abrasive being used, the air pressure, and by the distance and angle of the nozzle to the surface.
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