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Paint Coating

The Paint Coating Process

In order to achieve a quality, reliable and long-lasting coating, Surface Engineers follow good coating practices in line with recognized national and international standards. In short, the surface is prepared by abrasive blasting, which creates a surface profile which acts as an anchor for the coating. Surface preparation is to a coating system what a foundation is to a building. If a coating system has a poor foundation (Surface preparation) it will fail sooner than expected which can lead to great financial losses. Furthermore, any moisture on the surface will cause the coating to fail and therefore climatic conditions are measured prior to coating to prevent any failures. Single or multi-coat systems are applied in line with customer or paint manufacturer’s specifications. The wet film coating thickness is then measured, to check that it will cure to the specified dry film thickness. Finally, the dry film thickness is measured to ensure it complies with the specification.

Several very important processes are followed throughout the coating process:

Surface preparation: Abrasive blast to customer and/or paint manufacturers specification
Measurement of surface profile to customer and/or paint manufacturers specification
Climatic conditions (Dew point, relative humidity, surface temperature, air temperature and dew point) are measured prior to any coating being applied
Immediately after coating the wet film thickness is measured to check that the applied thickness will cure to the specified dry film thickness
After the coating has cured the dry film thickness is measured in relation to the customer and/or paint manufacturers specification
*Individual inspection for each layer for multiple coat applications.

What is the importance of these processes?

Surface Preparation

Surface preparation is the essential first stage treatment of a substrate before the application of
any coating. The performance of a coating is significantly influenced by its ability to adhere properly to the substrate material. It is generally well established that correct surface preparation is the most important factor affecting the total success of surface treatment. It affects the performance of the coating more than any other variable.

Surface preparation creates a foundation for the coating mechanically and chemically:

Mechanically: By providing an anchor/key for the coating
Chemically: By allowing intimate contact of coating material molecules with the steel (or other material) surface

Steel, when it is abrasive blasted, has a surface that is rough, with a series of tiny peaks and
valleys called surface profile. Coatings anchor to the valleys of the profile and the peaks are like
teeth. This is why the surface profile, created by blasting is called an “Anchor pattern” or a
“Mechanical tooth”.

The surface is blasted to standard grades of cleanliness in accordance with ISO 8501-1 1988 (BS
7079 Part A1 1989). This standard essentially refers to the surface appearance of the steel after
abrasive blast cleaning and gives descriptions with pictorial references of the grades of
cleanliness. Generally, Class Sa 2.5 is the most common:

Climatic Conditions

Optimal environmental conditions are essential for surface preparation, application, and curing
of coatings to maximize successful performance.

Surface preparation and the application of coatings should be performed under optimum
environmental conditions to help prevent failures. The following conditions should be observed:
Air temperature
Surface temperature
Relative humidity (RH)
Dew point temperature

It is commonly known that most coatings will not dry properly at low temperatures and high RH.
Surface moisture has a significant impact on the life and performance of the coating.Moisture forms on a surface when warmer, moist air comes into contact with it (condensation). Moisture will cause unprotected steel to rust. If it is trapped between a coating and a substrate, moisture will likely cause the applied system to fail prematurely.

Instruments are used  to help assess the risk of moisture forming on the surface. Tests are performed to calculate the dew-point temperature before, during and after the coating process. Dew-point temperature is compared to the surface temperature to ensure the two are far enough apart that moisture formation is unlikely.

Careful observation of atmospheric conditions and a good understanding of their impact on the quality and long term health of coating are essential.

Surface profile

The surface profile is specified independently from cleanliness. The surface profile is created by
the abrasive blasting. The correct height of the surface profile is essential, as it effects the
coatings overall performance. There is no standard surface profile height that is suitable for all

coatings. Basically, the surface must be roughened to get the coating to adhere to the surface.
This is specified by the coatings (Paint) manufacturer. Generally, thin coatings require a low
anchor profile. Too heavy a profile will result in the peaks of the profile in the steel sticking out
and causing pin point rusting. However, thick coatings require a deep anchor.

Surface Preparation Testing.

To ensure the correct level of surface Preparation has been achieved various tests are carried out
to the relevant International Standards using Calibrated equipment such as

Pictorial Standards

Pictorial Surface Standards are high quality photographs which are used for comparison purposes to
assess the visual appearance of a steel surface. Surface Standards covers most of those required for
surface cleanliness.

Surface Profile Gauge

Surface Profile Gauges are used to measure the profile height of a surface. The degree of the
surface profile on the surface affects a coating’s overall performance and determines aspects such
as adhesion, coverage and overall volume of coatings used. If the surface profile is too large the
amount of coating required increases, otherwise there is a danger that the peaks remain uncoated
– allowing rust spots to occur. If the surface profile is too small there may be insufficient key for
adequate adhesion leading to premature coating failure.

To ensure the correct surface preparation optimises the performance of the coating and material
usage, the profile height of a surface needs to be accurately assessed and measured. It is essential
to know this parameter as it determines the adhesion, coverage and overall volume of coating

Bresle Salt Kit.

It is essential that the level of contaminants on a surface is measured prior to application of the
coating to ensure the quality of the coating and that its optimum lifetime is achieved.

If the coating is applied to a contaminated surface, which is not properly prepared, it could fail
prematurely resulting in costly re-coating and high maintenance costs.

Dust Test Kit

Dust Tape Test kit allows assessment of the quantity and size of dust particles on surfaces
prepared for painting. Dust on blast cleaned surfaces can reduce coating adhesion, leading to
premature coating failure and sub-standard coating finish.