Surface Preparation consists of primary surface preparation and
secondary surface preparation. The primary surface preparation aims to remove mill
scale, rust, corrosion products, and foreign matter from a steel surface prior to
application of a shop primer or primer.
Blast Cleaning is based on the principle of an abrasive jet of
particles in a compressed air stream impinging on the surface, removing impurities,
millscale, rust and old paint. Abrasive blast cleaning is the most thorough and
widely used method of surface preparation in the shipbuilding and repair industry.
Different degrees of surface cleanliness are possible and depend in part on the
surface condition prior to treatment and also to the length of time for which the
surface is exposed to the abrasive jet. In addition to cleaning the surface, the
abrasive particles will impart a surface roughness to the steel.
Spot Blasting is localised abrasive cleaning often carried out
in ship repair, especially on the outside hull, where patchy corrosion or damage
has occurred. It can be used to yield surfaces that are cleaned to Sa 2 or better
but often surrounding intact areas are peppered with stray grit.
Water Jetting or Hydro Blasting as a surface preparation technique
is being used more and more in shipyards. A major advantage of using water pressure
as an abrasive is the lower impact on environment and health because less dust is
generated than is the case with grit blasting. It also constitutes less of a safety
risk caused by sparks and reduces the amount of salt remaining on the surface. As
with blast cleaning, prior to hydro blasting, water insoluble foreign matter such
as oil and grease must be removed.
Sweep Blasting is the treatment of a surface by quickly passing
a jet of abrasive across the surface. It is typically used as a tool to get some
surface roughness on an existing, firm adhering coating in order to facilitate inter
coat adhesion. The level of effectiveness depends on the skill of the operator,
the type of surface and particle size of abrasive. In general, a fine grade of abrasive
(0,2-0,5 mm) is recommended as larger particle sizes would destroy the existing
coating too much.