Surface preparation can account for up to 40 percent of structural steel painting and repainting jobs. As Rosler’s Structural Steel FAQ series has already established, the life of anti‐corrosion coatings on a steel surface depends to a large extent on how thoroughly the surface has been prepared for painting.
Properly evaluating the surface of structural steel surfaces for coating before and after shot blasting will help balance the cost of preparing, repairing, and monitoring structural steel throughout its impressive lifespan.
This installment of our Structural Steel FAQ series will answer How are rust and mill scale evaluated pre‐ and post‐blast?
Widely used standards were developed to visually assess the initial surface conditions and the quality of the required surface preparation relative to the initial steel surface conditions.
As an expert in the surface finishing industry, Rosler knows that all the expertise in the world won’t do any good if the surface of the work piece is not properly prepared.
When it comes to structural steel, we receive many questions about preparation. Among the most common questions is, “How is the presence of dust on shot-blasted structural steel components evaluated?”
Understanding dust considerations and mitigation will help produce higher quality and longer-lasting structural steel components more cost-effectively and safely.
The Dangers of Dust
Blast-cleaned structural steel surfaces must be completely free of dust to ensure proper coating and painting. Residual dust will reduce the adhesion of subsequently applied coatings and, by absorbing moisture, may promote the corrosion of the blast‐cleaned steel surfaces.
The potential accumulation of dust is especially critical on horizontal surfaces, the interior of pipes, and inside structural cavities. Special inspections must be carried out to ensure that such areas are adequately cleaned and free from dust before painting.
Besides the degree of cleanliness – the removal of oil and grease, rust and mill scale, dust, and other contaminants – surface preparation specifications must also consider the surface profile and roughness relative to the coating to be applied. Rosler Metal Finishing builds shot blasting equipment to create the right surface profile on structural steel components as well as cleaning them in preparation for coating and painting.
This installment of our Structural Steel FAQ series will answer What is the optimum the surface profile for a structural steel component prior to painting and how is profile evaluated?
Optimizing for Painting
Shot blasting makes a surface rougher to increase the total contact area between paint and a work piece substrate and generally improves paint adhesion. A surface that is too smooth poses the risk of inadequate paint adhesion, while a surface that is too rough may not cover the peaks. The degree of surface profile required depends entirely on the coating to be applied.
When designers and manufacturing companies specify surface finishes more often than not we’re talking in micrometres (µm), but what exactly is a µm?
1 µm is 0.001mm or one thousandth of one millimetre. In other words it is a very fine measurement and as such it can be hard to comprehend. So the best way to understand it for many of us is to compare it to everyday things we’d know, such as a cigarette paper, human hair, paperclip wire or needle.
This image from our partners Haas illustrates how these items measure up and what a µm equates to in comparison. For more details on their range of machining centres please visit their website at: http://www.haas-schleifmaschinen.de/en/
In comparison to microinches (written as µin or µ”) 1 µm = approximately 40 µ”. 1 µ” being equal to one millionth of an inch (0.000001”). (So a 15µm surface finish = 600µ” and a 320µ” surface finish = 8µm as examples of conversions).
Shotblasting of steel plate, profiles, construction, and fabrications is a process to clean, descale, provide a specified surface profile and edge break as a surface preparation. This process takes place prior to a coating or paint application to maximise the adherence potential and corrosion control.
This is a document made up of five parts on “How Do You Make Your Coatings Stick Better?”
Often engineering drawings call for a range of surface roughness. Traditionally this is a roughness value expressed in Ra although other measurement types such as Rz are becoming more common. But what do they mean?