Surface Preparation

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In The News


(Anchor Pattern) SURFACE PROFILE

Deep Valleys, Protecting the Peaks

Anchor pattern is a property of metal surfaces.  It is the roughness which is inherent to the metal.  Such a surface is never smooth. The more corroded the surface, the more pronounced its roughness. Every steel surface contains craters, cracks and pores.  Its pattern will affect the mechanical bond between surface and primer and between subsequent topcoats.

If we observe a steel panel that has been prepared / sand blasted, we see contours revealing peaks and valleys, as shown in Figure 1.  If a paint / coating is applied over such a surface, it will fill the valleys, but not necessarily cover the pinnacles.  

Roughness is not always visible to the human eye without proper magnification. If such a surface is excessively rough, this means the "top and valley structure" is very pronounced.  This is itself would not seem to be a disadvantage, as mechanical anchoring is improved by a coarse substrate, provided it is clean.

If the surface is rusted, it might still be full of crevices, Rust is often sponge-like, saturated with moisture, and contaminated with chlorides and sulfates.

Figure 1 shows from left to right three profiles from blasting, and wire brush.


Even when the surface has been cleaned manually, or by power tools, remnants of rust usually remain in the cavities.  Therefore, surface preparation is more effective when blast cleaning methods are used.

Whatever the blasting technique, it certainly will contribute to the roughness of the metal substrate as it removes the rust.

When the "brush-off' blast is used, a considerable amount of tight rust and mill scale could remain on the surface.  A commercial blast will be more satisfactory, except in such cases when subsequent exposure is severe.  For such highly corrosive conditions, blast cleaning to white metal is warranted, though the costs are higher.

In all these blast cleaning applications, the impact of the abrasive particles (sand, grit, iron shot, etc.) will determine the anchor pattern profile.  Their effect differs due to the particle size of the various abrasive materials.

One of the major drawbacks of mechanical blasting is the type of substrate it produces.  Such a surface is extremely reactive and, therefore, highly susceptible to corrosive attack.

A steel surface, blasted to white or near-white grade, should be promptly painted.  Otherwise, it will soon be covered by a powder-like rust blush.

Clearly, a good paint must cover the highest peaks in the anchor pattern.  If that is not the case, the tops will not be protected by paint. This leads to a surface full of rust and corrosion. If the dry film that covers the peaks is thin, it will leave the metal unprotected.    

This situation will be aggravated if the paint is later exposed to severe attack by moisture, chemicals, or other corrosive environments.  Under normal circumstances, a film thickness that exceeds the anchor pattern depth will prove sufficient.

The Void Beneath

The selection of the right blasting media will determine what the anchor pattern profile will look like.  The coarse grades are labor-saving because they clean the steel surface at the fastest rate. But as they also produce deeper profiles. For example, a 40-mesh sand abrasive produces an anchor pattern depth of 2-3 mils. Clearly, such a deep cut blast pattern will demand higher film thickness and the spreading rate of the paint will also be affected because the blast has increased the surface area.

Please contact Dampney Technical / Customer service for surface preparation and a coating recommendation.

Dampney Co. Inc.

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