The most common cause of premature coatings failure is improper ANCHOR PATTERN.

What is anchor pattern?  It is a term used to describe the roughness (etch, or profile) that is created on a surface when sandblasted.  The Anchor Pattern is measured in 1/1000th of an inch, which is called a MIL.  Paint coatings are also measured in MILS as well, making it a term that is easy to remember.

Coatings require specific mils of anchor pattern in order to properly adhere to the surface it is applied to.  Each coating will have different requirements.  Therefore, you cannot simply sandblast a surface clean and then apply paint to it without technical information on what mils profile that particular coating requires (among other factors).  If you simply guess at what profile you need, you risk premature coatings failure.

If you have no access at all to the coating manufacture’s technical data and need to make an educated guess, you will want to consider the following rule of thumb:

The mils of anchor pattern you create on the surface to be coated, should be 25 to 30% of the DRY film thickness of the total coatings system BUT NEVER greater than the dry film thickness of the primer coat unless additional coats are to be applied immediately.

Also key to understand is that the wet mils of a coating will be different than the dry mils of a coating.  Many coatings shrink tremendously, depending on the percentage of solids contained within them.  Make sure that the anchor pattern you create when sandblasting is much shallower than the total coatings when dry.

Other factors that impact the development of anchor pattern include the type of steel you are blasting (including hardness and chemical composition), how the steel was formed, prior use of the steel, the type of abrasive you use to create the anchor pattern (including size, shape, hardness, and velocity), as well as what type of blast nozzle you choose to use and how you use it (such as angle and distance to the work surface).

The chart below is a crude approximation of abrasive size to anchor pattern, and should thus be used only as a starting point only for both centrifugal wheel and pressure blasting.

• 1 Mil Profile = G80 Steel Grit, 100 Mesh Garnet, S110 Steel Shot, 3060 Coal Slag
• 1.5 Mil Profile = G50 Steel Grit, 100 Mesh Garnet, S170 Steel Shot, 3060 Coal Slag
• 2.0 Mil Profile = G40 Steel Grit, 3060 possibly also 80 Mesh Garnet, 36 Grit Aluminum Oxide, 3060 Coal Slag, 3060 Copper Slag
• 2.5 Mil Profile = G40 Steel Grit, 3060 Mesh Garnet, 24 Grit Aluminum Oxide, 2040 Coal Slag, 2050 Copper Slag
• 3 – 4 Mil Profiles = G25 Steel Grit, 36 possibly also 3060 Mesh Garnet, 16 Grit Aluminum Oxide, 2050 or 1230 Copper Slag, 1240 Coal Slag