(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.
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
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
and the spreading rate of the paint will also be affected because the blast has increased the surface area.
Dampney Technical / Customer service for surface preparation and a coating
Dampney Co. Inc.