Surface preparation refers to the elimination of dirt, debris, paint, rust, concrete..or other contaminates leaving the surface in an ideal state for the application of future coating or bonding.
Surfaces:
Steel: removes rust, dirt, concrete, debris and other contaminates. Sandblasting new steel removes the mill scale. Blasted surface leaves profile.
Concrete: removes salts, form marks, latent cement, discoloration, expose aggregate, mildew, staining, oil, graffiti, efflorescence and other contaminates.
Brick: removes dirt, debris, mortar, soot, paint (graffiti) and other contaminates.
Wood: removes layers of grain, soot, paint, debris and other contaminates.
Fiberglass: removes top layer of gel coat to expose air bubbles.
Aluminum and other metals: removes rust, dirt, and debris leaving surface profiles.
There are various abrasives to choose from. Depending on the surface which is to be prepared. For example when sandblasting wood sandblasters may choose crushed glass. The impact of angular abrasive roughens a surface to produce a profile.
What is a profile? Profile refers the scratch depth that the abrasive leaves on the steel surface. Abrasives cut into the steel forming peaks and valleys. The depth of the profile is controlled by the size of the abrasive, angle of nozzle, distance of nozzle to surface and compressed air flow. The profile provides a mechanical method of positive, uniform bonding between the coating and the surface.
Coating manufacturers test their coatings on various profiles making sure that their products perform as intended. Adhesion and performance of coatings are guaranteed if specifications are followed. Contractors must adhere to these specifications. When profiles exceed the maximum specifications, the peaks may protrude through the coating system, causing it to fail.
Steel surface preparation standards measure two critical specifications : Surface profile (discussed above) and Degrees of Cleanliness.
The Structures Painting Council (SSPC) http://www.sspc.org
Degrees of Cleanliness
The steel Structures Painting Council (SSPC) has established four degrees of cleanliness for blasting, ranging from removal of all contaminants to removal of loose materials only. The four degrees are White Metal Blast, Near White Metal Blast, Commercial Blast, and Brush off Blast For detailed descriptions of each refer to the SSPC's Visual Standards for Abrasive Blast Cleaned Steel.
White Metal Blast
Viewed without magnification, a shite metal blast cleaned surface is free of all visible rust, mill scale, paint, and foreign mater. This degree of cleanliness is usually required where sophisticated points; such as, zinc-rich coatings, will be applied to surfaces exposed to highly corrosive environments. Chemical plants, offshore drilling rigs, and bridges over salt water are typical applications.
Near White Metal Blast
Viewed without magnification, at least 95% of the surface is free of all visible residue after blast cleaning. This is similar to White Metal but some slight staining on the metal is allowed. This degree of cleaning is required where high performance coatings will be applied to steel exposed to harsh elements and heavy usage.
Commercial Blast
Viewed without magnification, at least two thirds of a commercial blast cleaned surface is free of visible residue. It allows some tightly adhering old paint to remain, based on the premise that paint adheres after blasting is unlikely to fail. For most applications where standard coatings will be applied, commercial blast is specified.
Brush Off Blast
Viewed without magnification a brush off blast cleaned surface may have only tightly adhering residues of mill scale, rust, and coatings remaining and must have numerous evenly distributed flecks of underlying metal exposed. This method is acceptable for surfaces not subjected to sever environments or where long-term coating life is not expected.
|
Degrees of Cleanliness |
SSPC Std. |
NACE Std. |
SIS Std. |
|
White Metal Blast |
SSPC-SP 5 |
NACE No. 1 |
SA-3 |
|
Near White Metal Blast |
SSPC-SP 10 |
NACE No. 2 |
SA-2 1/2 |
|
Commercial Blast |
SSPC-SP 6 |
NACE No. 3 |
SA-2 |
|
Brush-off Blast |
SSPC-SP 7 |
NACE No. 4 |
SA-1 |
