Selection of maintenance coating system is more complicated
than for new construction. Climatic conditions available time,
budget, surface preparation method and conditions tend to influence
a great deal in the selection of a maintenance system.
To avoid
coating failure, the maintenance system must be compatible
with the original coating system on the structure involved.
This will be unnecessary if conditions permit complete blast
cleaning to Sa 2,5 in which case an entirely new system can
be used.
METHODS OF SURFACE PREPARATION
1. Wire
Brushing: This is carried out by means of rotating
wire brushes. Suitable for the removal of mild scale and
thick rust. Tend to polish and rub in the surface with grease,
oil and corrosive products. This contamination causes poor
adhesion of primers, This is not recommended if it can be
avoided.
2. Chipping: This method involves pounding of the surface
with hammers to break up scales; thick rust and old paint
films. It is not suitable for surfaces that are to be painted
with epoxy chlorinated rubbers. Surface tolerant primers
e.g. Epoxy Aluminum are the best primers for this type of
surface preparation.
3. Disc
Sanding: Rotating disc covered with abrasive materials
is used. Good for local repairs.
4. Flame
Cleaning: Very high temperature flames are to treat
surface to be coated. This method removes mill scale but
much less rust. Not a good method for general preparation
or even removal of rust. There is tendency for rust promoting
compounds present in rusty surface to be passivated by the
high temperature attained during cleaning, also there are
inherent risk of fire and explosion.
5. Blast
Cleaning: This is the most effective method of
surface preparation. Whether either sweep blasting or sand
blasting the method involves the impact of high velocity
object on surfaces remove all corrosion products and contaminants.
The method could be centrifugal, vacuum or compressed air
blast clearing. Grit blasting particles should be free from
halves and shot blasting particles should be spherical and
solid with kittle or all tails.
6. Wet
Blast Cleaning: This method incorporates water into
the blasting process. It could be high pressure water cleaning,
low pressure water abrasive blast clearing, low pressure
humidified abrasive blast cleaning and steam. Wet blast cleaning
has the advantage of reducing dust and avoidance of spark
hazards. Results from low pressure water (humidified) abrasive
cleaning is comparable with dry abrasive blast cleaning.
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