Surface Cleanliness

10.13.3.5 Surface Cleanliness

Do not allow surfaces prepared for coating Surfaces must be free of oil and grease

to deteriorate or to be contaminated in any before blast cleaning. Blast cleaning does

way between the end of the cleaning phase not remove oil, grease or non-visible con-

and the beginning of the coating application. taminants. Inspect all surfaces after cleaning

Clean grit-blasted surfaces should not be to ensure they comply with the specification.

handled or touched unless clean protective Cleanliness after preparation is important, so

gloves are worn. Do not expose cleaned sur- ensure residual traces of abrasive are blown

faces for prolonged periods or expose them off, vacuumed, or swept away before primer

at all to high humidity either in the open or coating.

10-46 Surface Preparation

in storage. Under such conditions, oxidation There are so many variables that influence and rusting may proceed very rapidly.

cleaning that rates of cleaning cannot be stated definitively. These variables include,

As far as practicable, storage after blast

but are not limited to:

cleaning should provide a warm, dry envi- ronment. Apply coating as soon as possible

• Air availability

after blasting. Commonly observed rules

• Nozzle size and type

indicate a maximum delay of four hours

• Type of equipment used

after blasting and require that if visible dete-

• Condition of surface to be cleaned

rioration has occurred, the surface prepara- tion shall be repeated.

• Surface cleanliness standard required • Limitations on operator maneuverability

Inspectors should be aware of and consider all the air supply issues that can lead to inad-

• Lighting quality

equate surface preparation and surface con-

• Distance of nozzle from surface

dition at the time of coating. Some of the

• Skill of operator

things to be aware of are:

• Type and size of abrasive • Inadequate air supply. Theoretically, a

#6 nozzle (3/8 in [9.4 mm]), a popular

10.13.4 Inspector’s Checklist

size, requires 232 cfm (6,570 L/min). To

Always have a checklist that indicates the

provide the necessary quantity of air, a

proper inspection procedure items. The fol-

compressor of at least 300 cfm (8,600 L/ min) is normally used. A larger nozzle

lowing list includes some of what to focus

requires more air and a proportionally big-

on and monitor during dry abrasive surface

ger compressor.

preparations:

• Too small air hoses. Resulting friction • Ambient conditions losses are expensive.

• Conditions of substrate (rate or amount of • Internal hose couplings. Can cause up to

corrosion)

15% efficiency loss of the working pres- • Pre-blast surface cleanliness (oil, grease, sure. External couplings and nozzle hold-

dirt, etc.)

ers are a must. • Shot/grit size selection (IAW specifica-

• Badly designed machines. May have sig-

tion)

nificant pressure losses. • Shot/grit cleanliness

• Too small piping on the machine. Causes friction losses.

• Condition of abrasive blasting equipment • Compressed air lines not kept straight

• Surface profile (IAW with specification/ and as short as possible.

standards)

Other important factors affecting blasting

• Surface cleanliness after abrasive blasting

performances are:

(dirt, airborne contaminants, etc.) • Operator qualifications (know the opera-

• Correct choice of nozzles. Venturi nozzles tors and their abilities) are much more efficient than straight-bore

nozzles.

• Safety

• Clean, dry air.

Surface Preparation 10-47

10.14 Centrifugal Blast Cleaning

aircraft carrier, or on concrete surfaces,

The most complex of the centrifugal blast

such as floors, pits, etc.

cleaning cabinets ( Figure 10.76 ) are

• To eliminate the need for compressor set-

designed to blast large quantities of steel on

ups including air/blast hoses, an abrasive

a regular basis, such as all plate received by pot, and a pot attendant

a shipbuilding yard. These machines, often

• For lower production costs

known as wheelabrators, are designed to

• On work pieces such as pipe, piling, rein-

work on a continuous basis and include a

forcing steel, beams, flat plate, etc., with a

conveyor system to carry items through the

series of centrifugal blast wheels housed

cabinet continuously. It is typical for these in a blast enclosure, arranged so all sides

of the work are cleaned as it travels

cabinets to use a system of rotating wheels

through the equipment.

with vanes to propel the abrasive. (The term Wheel blast operations are described in

wheelabrator has been adapted for general more detail in CIP Level 2.

use.) These cabinets usually also have an abrasive recovery and recycling system and