Why Surface Preparation
The most important part of applying any coating system is preparation of the underlying substrate. If the surface preparation is poor then the performance of the coating is adversely affected in both performance and lifespan.
There are two reasons for surface preparation:
1. To ensure the surface is clean of oil, grease, paint or any kind of dirt that may adversely affect the bond between the coating and the substrate.
2. To provide a surface that will assist coating to bond better and thus last longer and be harder to remove.
Surface contaminants interfere with the mechanical bond between the coating and the surface either by preventing penetration or providing a layer between the adhesion and the substrate which is has a weaker bond and thus the coating will be compromised
Initial surface cleaning may be via a pressurised system that will remove any previous coatings and visible contaminants either by washing them away or blasting them from the surface with abrasion. For newly prepared surface it may take the form of a chemical solvent which will remove any non-visible contaminants that may be forming a film over a raw surface.
To assist the bond of a coating and a surface the surface must have a microscopic anchor pattern. Ultra smooth surfaces like glass do not bond well with coatings because the surface area of the bond is flat and thus can be sheared away easily. A saw-tooth pattern on the surface makes a much stronger bond with a coating as the coating seeps into the gaps anchoring the coating against shearing away from the surface.
Abrasive Blasting of Steel Structures
Abrasive blasting is an effective means of both removing surface contaminants like paint, protective coatings and rust and creating a roughened surface that will adhere well to new coatings.
Abrasive blasting is mechanical cleaning by continuous impact of abrasive particles at high-velocity against steel. There are various grades of abrasive blasting defined by standards which are required depending on specification for use. These specification cover both the cleanliness and the depth of the anchor profile.
The standard grades of cleanliness for abrasive blast cleaning in accordance with ISO 8501-1 are:
- Sa 1 – Light blast cleaning
- Sa 2 – Thorough blast cleaning
- Sa 2½ – Very thorough blast cleaning
- Sa 3 – Blast cleaning to visually clean steel
Specifications for bridge steel work usually require either Sa 2½ or Sa 3 grades.
There are a wide range of potential abrasive materials from aluminium oxide to steel shot and grit. These can be used to rejuvenate the steel by removing any rust, scale and dirt and bring the metal back to bare metal state to facilitate the better adherence of a protective coating.
At connections that are welded, the key components in ensuring the effectiveness of the coating system are the effectiveness of the protection before final coating. The areas local to welds are typically hidden, to prevent them being coated. The masking stays in place until the joint is welded; this isn’t an ideal form of protection if there is prolonged exposure before welding.
After welding, it’s essential that the joint surfaces, like the weld itself, are prepared to the specified standard of cleanliness and profile. Due to the pollution that occurs in the welding flux, particular attention must be paid to cleaning off all deposits.
Rough profiles, badly formed start-stop, sharp undercut and other defects including adherent weld spatter, should be removed by careful grinding. Particular focus must be paid to the blast because weld metal is harder cleaned profile and onsite blast cleaning is more difficult than shop blasting.
For more information on surface preparation see http://www.steelconstruction.info/Surface_preparation