The Purpose of Cleaning the Steel
A clean steel surface is necessary for a more uniform weathered appearance. Mill scale and other foreign matter should be removed so that the steel surface is uniformly exposed to the atmosphere. This is particularly important if a uniform weathered appearance is desired as early as possible. The mill scale on Weathering Steel is much tighter than mill scale on ordinary carbon steels.

Removal of mill scale will require a greater effort than with plain carbon steel. Areas which are not properly cleaned will not oxidize in a uniform manner. The result will be a mottled appearance for a period of several months or even years, depending upon degree of exposure and the local environment.

Cleaning is required, however, only if the initial appearance of the steel is important, since the long-term corrosion properties of the steel are not measurably affected by surface preparation.

How Much is Enough?
For most applications, "Commercial Blast Cleaning" (SSPC-SP 6) is sufficient to provide somewhat uniform weathering of the fully exposed steel. "Blast Cleaning to White Metal" (SSPC-SP5) will provide a better surface than "Commercial Blast Cleaning," but will cost significantly more. "Blast Cleaning to White Metal" is not generally required and should be considered only where a high degree of uniformity is necessary during the early stages of weathering.

"Near-White Blast Cleaning" (SSPC-SP 10) provides for an intermediate level of surface preparation. This specification suggests that the overall blast-cleaning effort expended should be not less than two-thirds nor more than 95% of that required to produce a white metal finish on the same surface.

In some instances, it may be necessary to remove heavy coatings of oil or grease with a suitable solvent prior to the blast -cleaning operation. Removal of the oil and grease permits more efficient blast cleaning.

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