Surface Preparation is the process of cleaning, decontaminating, and roughening a substrate before a coating is applied. It consists of removing contaminants that could result in premature failure of the coating and achieving specified levels of cleanliness in accordance with technical standards for each coating to be applied.

A proper surface preparation procedure should include these 3 steps:

* ⁠decontamination

* cleanliness

* ⁠surface profile

 

1. Decontamination:

If the presence of oils or greases is determined, appropriate detergents or cleaning products must be used for chemical cleaning.

Water should also be used to remove any residues of cleaning agents as well as reduce the level of non-visible contaminants such as chlorides and salts.

The goal is to remove from the surface any visible and non-visible contaminants that could interfere chemically or mechanically with the coating material and cause premature failure.

Types of Contaminants

Visible:

- Oil and grease

- Dust and dirt

- Cutting compounds

- Other contaminants

Removal:

- Solvents

- Detergent

- Hot water

- Steam

Non-visible:

- Soluble salts

- Sulfates

- Nitrates

- Chlorides

Removal:

- Fresh water washing

- Water blasting

- Wet abrasive blasting

- Steam cleaning 

 

2. Cleanliness:

This is a visual standard that determines the percentual amount per area of visual contamination present on the surface of the substrate, each coating and / or application may have different cleanliness requirements.

CLEANING STANDARDS AND TECHNICAL ASPECTS FOR GREATER SURFACE PREPARATION EFFECTIVENESS

As a reference here are some of the most common examples of cleanliness levels according to the ISO 8501 & the joint SSPC SP / NACE # norms:

Brush Off / sweep blast

- SP 7 / Nace #4 / Sa 1

- Loosely-adhering material: 0%

- Tightly-adhering material: 100%

- Stains, streaks, shadows: 100%

Brush Off is specified to remove loose rust, mill scale and coatings, and uniformly roughen up a surface in preparation for a new coat. Tightly-adherent materials are permitted to remain.

Industrial Blast Cleaning

- SP 14 / Nace #8

- Loosely-adhering material: 0%

- Tightly-adhering material: 10%

- Stains, streaks, shadows: 100%

Industrial is specified for conditions when the existing coating is thin, well-adherent and compatible with new coating. It is the most recent abrasive blasting standard, and not widely specified. ISO has no corresponding specification.

Commercial Blast Cleaning

- SP 6 / Nace #3 / Sa 2

- Loosely-adhering material: 0%

- Tightly-adhering material: 0%

- Stains, streaks, shadows: 33%

Commercial is specified when a high, but not perfect, degree of cleanliness is warranted. It’s common for repainting products that serve in non-corrosive environments and atmospheres, such as tanks and bridges.

Near White Blast Cleaning

- SP 10 / Nace #2 / Sa 2.5

- Loosely-adhering material: 0%

- Tightly-adhering material: 0%

- Stains, streaks, shadows: SP 10 5%, Sa 2 ½ 15%

Near White is typically specified for high performance coatings over steel exposed to severe environmental conditions, such as chemical spills and fumes, high humidity, and proximity to salt water. It is commonly specified for off-shore platforms, shipyards and other marine environments.

White Metal Blast Cleaning

- SP 5 / Nace #1 / Sa 3

- Loosely-adhering material: 0%

- Tightly-adhering material: 0%

- Stains, streaks, shadows: 0%

White metal is specified for steel serving under high temperatures, high pressures, and corrosive environments, and in cases where the catastrophic consequences of coating failure justify the extra expense, such as nuclear reactors, turbines, chemical tank linings, submarines, etc.

 

3. Surface Profile:

During the surface preparation process, it is critical to consider the proper profiling, which involves roughening the surface to create the necessary surface profile to achieve the correct adhesion of the coating.

One of the most common methods of profiling involves projecting abrasives under high pneumatic pressure against a surface, to achieve the desired effect, be it cleaning, removing impurities, oxidation, old paintwork, and of course the adequate profile. This method is known as “Abrasive Blasting”.

Other known methods are manual mechanic tools, Wet Blasting, Ice Blasting, and HPWJ, among others.

The most common abrasives are:

• Aluminum Oxide
• Steel Shot
• Sand
• Baking Soda
• Ground Glass
• Copper Nickel Slag

Choosing the correct abrasive is essential to achieve the desired result effectively and safely. With the wide range of materials available, the possibilities are extensive, adapting to almost any industrial need.

The profile is presented as the measure in mils or microns between the peak and the valley of the roughened surface. Its requirement will be determined by each coating manufacturer by establishing the minimum contact area required by their product and their product’s weight and characteristics to achieve its maximum level of mechanical adhesion

The lack of appropriate preparation of the surface where the coating will be applied is the cause of 75% of failures in coatings and repairs.

The improvement and application of the standards required by certification bodies become fundamental for the effectiveness of maintenance, whether corrective or preventive, in industrial facilities and systems.