Stages in paint inspection
During a construction phase, inspection should be carried out at
the following critical stages:
Inspection of steel to be used
Inspection of applied shop primer
Inspection of steel work (welding, cutting of edges)
Inspection of surface preparation
Inspection before and during application
Inspection after application
INSPECTION OF STEEL WORK
When a new construction is built or when maintenance work
includes steel exchange or blast cleaning, the steel work must be
checked for:
Rounding of sharp edges
Smoothing of rough welding seams
Removal/grinding of weld spatter and beads
Cracks and pitting
Surface faults like laminations, etc.
INSPECTION OF SURFACE PREPARATION
Inspection of surface preparation includes the following:
Cleanliness; solvent cleaning to remove salt, oil, grease,
and dust/dirt
Evaluation of present condition (rust grade)
Evaluation of surface (preparation grade and roughness)
Remaining contamination acceptable?
Rust grade
The ISO 8501(-1) standard is an important tool to inspect steel work and surface preparation. It
gives four rust grades which are used to assess the steel surfaces. These are given the
designations A, B, C and D. Photographs are used to illustrate the four rust grades of the steel.
As well as photographs of dissimilar rust grades A, B, C and D, the standard also contains 24
pictures which show the visual cleanliness after mechanical pre-treatment by wire-brushing,
blast cleaning and flame cleaning on steel substrates originating from the four different rust
grades.
Roughness
There are many test methods to measure roughness of the surface. Among the most suited for
the field are comparators. In ISO 8503, two comparators are
specified; one with profiles corresponding to blast cleaned
surfaces using grit abrasive (reference comparator G) and
one corresponding to blast cleaned surfaces using metallic
shot abrasives (reference comparator S).
The nominal values for these profiles, identify the limits of
the three grades fine, medium, and coarse.
Contamination
An easy test for detecting oil/grease on a surface is the "water
break method", where a drop of water is added on to the prepared surface. The drop will spread out
rapidly on the surface if no oil/grease is present, but will remain on the surface in a drop-shaped
form in the presence of oil/grease. This method is not a standard.
To detect water-soluble salts, the most user-friendly method available today is to dissolve the salts
that are present at the surface and to measure the conductivity of the water sample. Conductivity is
an indication of how well a liquid solution will conduct electricity and is measured in micro-Siemens
(µS). The conductivity can through calculation be converted to a corresponding salt content on the
surface. A test method for determining the total amount of soluble salts is described by two
standards:
The sampling is given by ISO 8502-6, "Extraction soluble contaminants for analysis", the
Bresle sampler.
The analysis is described by ISO 8502-9, "Field method for soluble salts by conduct metric
measurement".
INSPECTION BEFORE AND DURING APPLICATION
Inspection during application includes the following activities:
Climatic conditions
Technical Data Sheet must be available and followed
Ensure correct mixing and thinning (extremely important)
Measuring the wet film thickness (WFT)
Number of coats as given in the specification
Cleanliness between coats (salts, dust, oil etc.)
Drying time between coats, minimum and maximum
The workmanship
Controlling the equipment and methods used
Wet film thickness
The wet film gauge (ISO 2808) is a useful tool to measure wet film
thickness on flat, even surfaces. When measuring the second coat, a soft
first coat or, if the first coat is of a resoluble paint type, the results may
be inaccurate.
Relative humidity
The sling hygrometer (ISO 8502-4) consists of two thermometers, one
dry and one wet (wet cotton wool wrapped around the sensor). On
rotation, the water in the cotton wool will evaporate, thus cooling the
thermometer in ratio to the dry one, which measures the temperature of
the air. From the temperature readings, the relative humidity can be
calculated.
Dew point
The dew point is the highest temperature at which moisture will
condense from the atmosphere. The dew point is essential to determine
if the climatic conditions are acceptable for paint work. The calculation
can be done from tables or by a so-called dew point calculator. There are
two slightly different types available today. They are based on similar
principles and consist of two seals, which are set against each other, so
that the required information can be read. When you have measured the
dry and wet bulb temperature, the dew point and relative humidity can
be read from the dew point calculator Dew point
The dew point is the highest temperature at which moisture will
condense from the atmosphere. The dew point is essential to determine
if the climatic conditions are acceptable for paint work. The calculation
can be done from tables or by a so-called dew point calculator. There are two slightly different types
available today. They are based on similar principles and consist of two seals, which are set against
each other, so that the required information can be read. When you have measured the dry and wet
bulb temperature, the dew point and relative humidity can be read from the dew point calculator
Steel temperature
The contact thermometer is an electronic instrument to measure the
steel temperature. When steel temperature is measured and you have
found the dew point, you can determine if it is possible to start the paint
application. The steel temperature should always be 3 oC above the dew
point.
INSPECTION AFTER APPLICATION
Another critical stage that needs to be followed up is when the application has been finished.
Inspection after completion of the application includes:
Dry film thickness (DFT)
Curing/drying
Adhesion
Holiday detection
Dry film thickness
The most used instrument in the field is the electromagnetic dry film
thickness gauge. A magnetic dry film gauge is a simpler instrument
which gives less exact measures. It is important to calibrate the
instruments to zero and thickness similar to be measured.
Electromagnetic dry film gauges are available in many versions with
different kind of additional functions.
Adhesion
The pull-off test (ISO 4624): Dollies are connected to the painted
surface with glue. The force needed to loosen the dolly, is measured
by the instrument. Several types of pull-off instruments are
available. The test is a destructive test, which means that the test
will damage a small part of the paint film.
