Quality Grades on Blasted Steel
Starting with the top right picture and rotating counter clockwise, the four pictures represent grades "D", "C", "B", and "A". In the blasting industry, price sometimes becomes the single largest factor in weather a company wins a bid or loses a job to the competition. It is therefore understandable that most sandblasting companies price their work based on grades "D", and "C". These grades of quality leave much to be desired, however. Commonly referred to as a commercial blast, these grades don't come close to the quality standards of a white metal blast. In the end there is no way to tell what's under your paint, primer or powder-coating until the stuff starts peeling, chipping, or rusting straight through the coating. Then of coarse, it's to late to worry about.
It is an established fact that the better the grade of blasting you obtain, the longer the coating you apply is going to last. It's all about adhesion, or lack there of. A glossy surface will provide less surface area at the substrate level than a dull or flat matte finish. By abrading the surface of a substrate with blasting, one creates a greater amount of surface area and the surface becomes more and more dull. During sandblasting operations, metal removal begins once the paint or other coating is removed. This is the process at work in abrasive blasting with silicon carbide, aluminum oxide, silica sand, coal slag, copper slag, bar-shot, steel grit, crushed glass, garnet, and many other angular grits used in sandblasting.
All the above can provide a white metal grade "A" finish on metals. Glass beads, walnut shell, corn cob grit, plastic medias, and numerous other mildly abrasive and nonabrasive medias will remove coatings but will most likely not be capable of obtaining white metal finishes in all cases. They simply are not designed to remove medium to heavy corrosion. They all have their proper use and we use many of them for specialized surface preparations. However, a white metal finish will produce better adhesion between substrate and coating and thus, increase longevity.
A-1 prefers to blast everything to a white metal or near white metal surface unless a customer demands a lower quality or desires a lower price. Obviously time and price will be commensurate with the quality of any job. The typical difference between a commercial and white metal blast can be as much as double the cost, especially when encountering mill scale as pictured here or other heavy pitted rust. The choice is yours to make, but if you don't specifically demand white metal you're probably receiving a commercial blast and that can be the most costly job of all.