Surface Preparation is the most important part of a coating system - Series Part 2

The coatings inspector is the one ultimately responsible for ensuring that the specified degree of surface cleanliness is actually achieved.

Coatings experts often say that surface preparation is the most important part of a coating system.

Effective surface preparation is of paramount importance to the success and longevity of a coating system.

Surface preparation affects the performance of the coating more than any other variable. Given that the proper coating system has been selected, if the surface preparation is poor, coating performance is usually going to be poor.

If surface preparation is good, then the coating applied over it is likely to perform well.

For you, the applicator, it is useful to know the reasons why surface preparation is so important, because knowing why can help you to do a better job.

Surface Preparation Is a Foundation

First, we can express the reason for the importance of surface preparation in a broad, general way, with the help of an analogy or comparison.

Surface preparation is to a coating system what a foundation is to a building. If a building has a poor foundation it can collapse altogether.

If a coating system has a poor foundation / surface preparation), it will fail faster than expected (say, after 5 years rather than 10 years); or it can fail catastrophically, within the first year of application.

Both reduced service life and catastrophic failure, great financial losses occur to a facility owner.

The contractor may be held responsible for these losses if the surface preparation work is found to be faulty.

Surface preparation creates a foundation in 2 important ways: mechanically, by providing an anchor for the coating (adhesion-surface area) ; and chemically, by allowing intimate contact of coating molecules with the steel surface (bonding).

An inspector is likely to examine in the inspection of a coatings project following.

Scrutinize accurately the specification to make firm

1  the standard against which the work is to be measured;

2  the required level of surface cleanliness;

3  the maximum allowable surface profile;

4  special tests that may be required in the contract.

Identify the area being tested and

1  ensure that it is correctly identified in the daily inspection report and

2  ensure that the daily inspection report matches with the progress chart.

Inspect the pre-surface preparation condition of the steel to ensure that the requirements of the specification will be achieved.

Prior to blast cleaning, confirm that:

1  the surfaces are free from oil and grease;

2  the abrasive to be used is suitable and that it is recorded in the daily inspection report;

3  sieve analysis has been carried out, if necessary;

4  the abrasive is free from contamination, especially in cases of shop preparation where recycled media are used.

Beyond the obvious requirement that the abrasive should produce a clean steel surface that is free from contamination, it also should provide an adequate surface profile, a working mix of particle sizes, and particles only within the grading required.

Information should be obtained from datasheets about chemical analysis, hardness, density, chloride content, and conductivity. Conformity of the product to national or international safety regulations is typically indicated as well.

Carry out inspection of the prepared surfaces as necessary and when requested by the coating contractor’s supervisory staff to ensure that:

Carry out, when necessary, special tests (e.g., soluble salt, rogue peaks, etc.) required by the specification.

1  Overwhelming the adverse of dirt; dust; grease; oil; rust; moisture; and, in some cases, mill scale.

2  Overwhelming the adverse of Non Visible chemical Contaminants

The most dangerous forms of Non Visible contaminants are soluble salts: chlorides and sulfates.

When chlorides and sulfates are painted over, they are hygroscopic and draw moisture to cause detachment, blistering, and enhanced corrosion of the steel substrate lying beneath .

Blistering caused by underfilm corrosion is a more gradual process, taking many months to appear, but the presence of conductive salts under the paint film and/or in corrosion products will greatly accelerate this.

It is prudent to examine for the presence of soluble salts with approved field test kits before paint application and then to apply further cleaning stages for salts purging, if they are present in negative amounts.

Certain projects conductivity/soluble salt contamination testing is mandatory.

The results of osmotic blistering salt contamination has a significant effect on coating performance and it is critical that monitoring of salt levels on the substrate is undertaken in Cargo, Water Ballast, Potable and Grey Water tanks .

The International Maritime Organisation “Performance Standard for Coatings (PSPC) for Dedicated Sea Water Ballast Tanks in All Types of Ships and Double-side Skin Spaces of Bulk Carriers” specifies a maximum allowable water soluble salt substrate contamination level of 50mg/m2 (5 micrograms/cm2) expressed as Sodium Chloride.

Adhesion is a primary function of paint.

Wet and dry adhesion of a coating system are two very different things with adhesion being significantly decreased in immersed conditions.

Contamination and adhesion, depend exclusively on the method used to prepare the substrate before coating application.

Surface defects unnoticed for normal service, can quickly result in coating failure when painting tanks.

Delamination on an artificial scribe after salt spray exposure should be considered as a one of a set of measurable factors representative of the corrosion protection performance of coatings.

It is absolutely necessary that for immersed environments the highest standard of preparation is achieved, this is the reason Sa 2 ½ blast, or equivalent, is normally required for such applications.

Record the results of the inspection, confirming that the areas inspected may be recognized from the reports.

Where remedial work is necessary, make sure that:

1  the areas are appropriately marked up so that the contractor’s supervisor can recognize them;

2  the supervisor and workers are aware of any reasons for rejecting the work and the requirements for further work;

3  the contractor’s supervisor is informed in writing of remedial work; and

4  the area under preparation is rechecked to make certain that it meets the requisite standards.

·Measure and record the surface profile using the appropriate standard method.

·Ensure that the prepared surface is coated within the maximum over-coating intervals. Over-coating interval related problems are more likely to occur with coatings which cure by cross linking, such as epoxies or polyurethanes.

·Inspect the steel prepared surfaces and ensure that the specified standards have been achieved prior paint application

When the specification calls for surface preparation by mechanical cleaning methods, in addition to the above inspection requirements, it will be necessary to confirm that:

1  excessive burnishing of the metal is avoided; Polishing of actual steel surfaces may also be a problem if brushing is carried out over zealously, because smooth polished surfaces can be detrimental to coating adhesion and

2  any visible peaks produced by power tool cleaning methods are removed. If this type of surface is painted without further preparation, the rough peaks can protrude through the coating and cause rash rusting and coating failure