© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
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4<br />
Blast Cleaning and Other<br />
Heavy Surface<br />
Pretreatments<br />
In broad terms, pretreatment of a metal surface is done for two reasons: to remove<br />
unwanted matter and to give the steel a rough surface profile before it is painted.<br />
“Unwanted matter” is anything on the surface to be painted except the metal itself<br />
and — in the case of repainting — tightly adhering old paint.<br />
For new constructions, matter to be removed is mill scale and contaminants.<br />
The most common contaminants are transport oils and salts. Transport oils are<br />
beneficial (until you want to paint); salts are sent <strong>by</strong> an unkind Providence to plague<br />
us. Transport oil might be applied at the steel mill, for example, to provide a<br />
temporary protection to the I-beams for a bridge while they are being hauled on a<br />
flatbed truck from the mill to the construction site or the subassembly site. This oilcovered<br />
I-beam, unfortunately, acts as a magnet for dust, dirt, diesel soot, and road<br />
salts; anything that can be found on a highway will show up on that I-beam when<br />
it is time to paint. Even apart from the additional contaminants the oil picks up, the<br />
oil itself is a problem for the painter. It prevents the paint from adhering to the steel,<br />
in much the same way that oil or butter in a frying pan prevents food from sticking.<br />
Pretreatment of new steel before painting is fairly straightforward; washing with an<br />
alkali surfactant, rinsing with clean water, and then removing the mill scale with<br />
abrasive blasting is the most common approach.<br />
Most maintenance painting jobs do not involve painting new constructions but<br />
rather repainting existing structures whose coatings have deteriorated. Surface preparation<br />
involves removing all loose paint and rust, so that only tightly adhering rust<br />
and paint are left. Mechanical pretreatments, such as needle-gun and wire brush,<br />
can remove loosely bound rust and dirt but do not provide either the cleanliness or<br />
the surface profile required for repainting the steel. Conventional dry abrasive blasting<br />
is the most commonly used pretreatment; however, wet abrasive blasting and<br />
hydrojet cleaning are excellent treatment methods that are also gaining industry<br />
acceptance.<br />
Before any pretreatment is performed, the surface should be washed with an<br />
alkali surfactant and rinsed with clean water to remove oils and greases that may<br />
have accumulated. Regardless of which pretreatment is used, testing for chlorides<br />
(and indeed for all contaminants) is essential after pretreatment and before application<br />
of the new paint.<br />
<strong>©</strong> <strong>2006</strong> <strong>by</strong> <strong>Taylor</strong> & <strong>Francis</strong> <strong>Group</strong>, <strong>LLC</strong><br />
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