Blast cleaning, grit blast, sand blast or shot blasting?

We grit blast with aluminium oxide, which is a harder, more effective medium to use but similar in appearance to sand. Windridge Coatings has two walk in grit blast bays:

• Bay 1, blast medium = Steel grit, capable of accommodating 10m x 1.0m x 2.7m high.
• Bay 2, blast medium = Aluminium oxide, capable of accommodating 5m x 2.5m x 2m.

Paint chips off clean mill scale

Although clean mill scale looks like a good surface to paint on, as most fabricators know it is very hard and brittle. When attempts are made to remove it by grinding or abrading, it is often left polished and shiny, this makes an unsuitable surface to paint on. Should the component suffer an impact, the paint is likely to chip off the mill scale.

Mill scale chips off metal

This image shows part of a chair powder coated by another trade coater. We were asked to strip and re-powdercoat it because of chipping when it fell over. They had applied a thick coat of powder over mill scale, and as can be seen, the mill scale has chipped, taking the paint off with it. When powder coating is applied properly to a good key, it can sustain a hammer blow. The paint will smudge but not chip.

Caution should be used when dealing with a Powder Coating company that does not include grit blasting, as part of their specification when painting over mild steel for architectural use.

All thick gauge steel is grit blasted

As a matter of policy Windridge Coatings grit blasts all thick gauge steel to SA 2 1/2 (grey metal). This will produce an excellent key for the paint to adhere to. The finish will also be improved because; mill scale being so hard, chips leaving steps under the paint. Blasting will remove mill scale, rust and most contaminants; it will also reduce light scratches and blemishes.