Blasting:
By far the most effective method for removal of millscale, rust and old coatings,using abrasives such as sand, grit or shot under high pressure.

The grade of blasting suitable for a particular coating specification depends on anumber of factors, the most important of which is the type of coating systemselected.The primary standard used in the product data sheets in this manual is ISO 8501-1:1988(E), preparation of steel substrate before application of paints and relatedproducts - visual assessment of surface cleanliness. This standard represents a slightextension of the Swedish Standard (SIS 05 59 00), which was developed by theSwedish Corrosion Institute, in co-operation with the American Society for Testing &Materials (ASTM), and the Steel Structures Painting Council (SSPC), USA, and isalready used on a world-wide scale.Where appropriate, the nearest equivalent SSPC specification has been quoted onindividual product data sheets. It is recognised that the SSPC and ISO standards arenot identical, and as a consequence certain product data sheets may show gradeSa2½ (ISO 8501-1:1988) as equivalent to SSPC-SP6, (commercial blast cleaning),whilst others will be equivalent to SSPC-SP10 (near white metal). The selection ofthese blast cleaning grades will have been assessed using a number of factorsincluding coating type, performance expectation, and in-service conditions.As a general principle, where products are recommended for immersion oraggressive atmospheric conditions the blasting standard required will be to Sa2½ (ISO 8501-1:1988) or SSPC-SP10, however, when products are recommendedfor general atmospheric exposure the blasting standard required will be Sa2½ (ISO 8501-1:1988) or SSPC-SP6.Prior to blasting, steelwork should be degreased and all weld spatter removed. Ifsalts, grease or oil is present on the surface it will appear to be removed by theblasting process, but this is not the case. Although not visible, the contaminationwill still be present as a thin layer, and will affect the adhesion of subsequentcoatings. Weld seams, metal slivers and sharp edges revealed by the blasting processshould be ground down, as paint coatings tend to run away from sharp edges,resulting in thin coatings and reduced protection. Weld spatter is almost impossibleto coat evenly, in addition to often being loosely adherent, and it is a common causeof premature coating failure.
source: http://www.international-pc.com/pc/pds/pre_uk.pdf

Sweep Blasting:
In order to roughen the typically smooth galvanized surface after cleaning, an abrasive sweep or brush blast may be used. Care should be taken to prevent removing too much of the zinc coating. Particle size for a sweep blast of galvanized steel should range between 200 and 500 microns (8 to 20 mils). Aluminum/magnesium silicate has been used successfully in the sweep blasting of galvanized steel. Organic media such as corn cobs, walnut shells, corundum, limestone, and mineral sands with a Mohs hardness of five or less may also be used. The temperature of the galvanized part when blasting can have a significant affect on the finished surface profile. Sweep blasting while the galvanized part is still warm, 175 to 390 degrees F, provides an excellent profile. Ambient conditions for sweep blasting are recommended to be less than 50 percent relative humidity and a minimum air temperature of 70 degrees F.
source: http://www.corrosion-doctors.org/MetalCoatings/galvanized-cl...