Coatings shrink as they cure, resulting in low film thickness over sharp edges and welds, which is a classic cause of coatings failure.
Repairing defects is normally not part of an abrasive blaster’s job description, but when you are charged with prepping the surface it’s imperative the coating adheres properly, and surface defects can prevent that from happening. The abrasive blaster is the only person on the job who looks at every single square inch of the steel surface, which makes you the last line of defence for finding defects.
Finding and reporting defects is not only important, but welcome. Providing this additional service is a good way to make an impression and distinguish yourself from the competition. But first, you need to know what to look for.
Three Kinds of Steel Surface Defects
ISO 8501-3, “Preparation of Steel Substrates Before Application of Paints and Related Products”, sorts defects into three categories:
• Steel surfaces in general
When inspecting for defects, pay close attention to welded joints, cuts, punctures and scoring. Run a gloved hand over the distressed areas looking for places where it catches to identify protrusions. Some protrusions, such as weld spatter, can be abrasive blasted. Others will require grinding.
If you find yourself in the position where you are asked to grind a protrusion with power tools, it’s important to avoid leaving any surface roughing, burrs or burnishing. It’s also important to note when grinding defects to not reduce the mass of the substrate to less than that the surrounding metal, or grind in a way that creates excessive heat – both cases will weaken the steel. Also, grinded defects should be inspected afterwards by the glove test to ensure that more sharp edges were not created in the process.
Knowing what kinds of defects to look for, lets look at the different grades of surface prep.
Making the Grade
There are three grades for steel surfaces dealing with visible imperfections as set out by ISO 8501-3:
P1 Light Preparation: no preparation or only minimum preparation needs to be carried out before application of paint;
P2 Thorough Preparation: most imperfections are remedied;
P3 Very Thorough Preparation: surface is free of significant visible imperfections.
When deciding on the preparation grade, make sure you’re preparing the surface to specifications, especially considering that a single fabrication may require different prep grades — for instance: P1 on the inward-face and P3 for the outside wall.
Getting a consensus on between all relevant parties on preparation grades and specific visible imperfections before you start can save time and effort.
Imperfections, Defects and Prep Grades Revealed
During the transfer of wire to weld there is a disturbance in the molten weld pool. This disturbance, wusually caused by the voltage being too low or amperage being too high, causes weld-matter to fulminate onto the steel surface.
P1 - Fee of all loose weld spatter (a)
P2 - Free of all loose and lightly adhering weld spatter (a+b)
P3 - Surface must be free of all weld spatter. (a + b + c)
The deoxidization process between the flux coating, air, and surface; the result of which is deposited as a residue on and around the weld bead.
P1 - Surface shall be free from welding slag
P2 - Surface shall be free from welding slag
P3 - Surface shall be free from welding slag
Weld Porosity is a common welding defect. When the torch is applied to treated metal, gasses are released and absorbed into the molten metal. As the metal cools, the gasses ar released from the metal, forming pores.
1 - Visible
2 - Invisible (might open after abrasive blast cleaning)
P1 - No Preparation
P2 - Surface pores shall be sufficiently open to allow penetration of paint
P3 - Surface shall be free from visible pores
Edges: Punch, Shear, Saw, Drill
Edge transitions made during fabrication by punching, shearing, sawing or drilling tools.
P1 - No part of the edge shall be sharp; the edge shall be free from fins
P2 - No part of the edge shall be sharp; the edge shall be free from fins
P3 - Edges shall be rounded with a radius of not less than 2 mm
Pits and Craters
When corrosion, of an extremely localized variety, leads to small perforations on the steel substrate.
P1 - Pits and craters shall be sufficiently open to allow penetration of paint
P2 - Pits and craters shall be sufficiently open to allow penetration of paint
P3 - Surface shall be free of pits and craters
A layer of corrosion in steel causting the corroded surface to separate and lift, leaving an interlocking flaky shell texture.
P1 - Surface shall be free from lifted material
P2 - Surface shall be free from visible shelling
P3 - Surface shall be free from visible shelling
Roll Overs/Cut Laminations
A fabrication defect cutting into the surface causing a thin protruding slice.
P1 - Surface shall be free from lifted material
P2 - Surface shall be free from visible roll-overs/cut laminations
P3 - Surface shall be free from visible roll-overs/cut laminations
Rolled-in Extraneous Matter
A defect ocurring during the fabrication process where foriegn matter is caught under a mechanical roller, and embedded into the steel surface.
P1 - Surface shall be free from rolled-in extraneous matter
P2 - Surface shall be free from rolled-in extraneous matter
P3 - Surface shall be free from rolled-in extraneous matter
Groves and Gouges
A disfigured burrow or rough opening found in a steel surface, typically caused by mishandling.
P1 - No preparation
P2 - The radius of groves and gouges shall be no less than 2 mm
P3 - Surface shall be free from groves, and the radius of gouges shall be grater than
Surface defects should be identified during the inspection, with a plan to remove them discussed before the job begins. However, if you encounter surface defects that look problematic during the course of blasting, report the defects to the project manager before taking action.