What is Sand Blasting?
Sand blasting is the preparation of a substrate and is carried out before spray galvanizing / aluminizing.

Swedish Standard SIS 05 5900-1967: "Life of an anti corrosive coating is dependent on how thoroughly the surface has been prepared prior to coating".

Steel Structures Painting Council SSPC-PA 15213, USA: "Surface preparation is the single most important factor in coating performance".

Metal spraying and painting have long been regarded as competitive but it is rapidly being realized that this is entirely not correct. With paint alone ultimate failure invariably results from rusting of the underlying steel, once barrier protection has ceased. A sprayed deposit of Al or Zn provides positive protection by virtue that they are anodic to steel and sacrifice themselves. A combination with metallized coating covered with suitable paint provides the ideal treatment for long term protection. Maintenance repainting is then only necessary at very long intervals.

 

Surface Preparation: meticulous cleaning and roughening of the surface by sand / grit blasting has always been employed for metal spraying as laid down by British and Swedish standards. Most paint manufacturers often claim that their products can be satisfactorily applied to a rusted surface, others demand preparation to various degrees. There is however little doubt that cleaning to bare metal is essential for best results.

Spraying versus painting: sprayed metal coatings are applied in a molten state to form a pure metallic deposit in contact with the base. Paint solutions which have metal particles in suspension are applied without heating, with the result that the metallic particles in the solution remain in precisely the same form as in the original solution without forming the fine atomic spray as in sprayed metal coatings.

Electrolytic protection: prolonged protection can only be provided by metallic coatings which are anodic to the base metal and sacrifice themselves by cathodic action. Scientists have proved that a satisfactory degree of electrolytic protection from zinc rich paints requires a Zn content of at least 93% in the film. However at this percentage, the solutions become very thick and are not easy to apply uniformly either with a brush or by spraying. Moreover the pain loses its binding power at this level and even after continuous stirring, it is practically impossible to ensure that the percentage of Zn at every point will be at the required level.

Costs: despite various claims to the contrary, an adequate metallic paint system is not cheaper than a Zn or Al coating applied my metal spraying. Relevant factors to be considered here include the increased corrosion free life obtained by metal spraying compared to the periodic painting required at regular intervals and the longer downtime required in painting for drying.