During a construction phase, inspection should be carried out at the following critical stages:

• Inspection of steel to be used
• Inspection of applied shopprimer
• Inspection of steel work (welding, cutting of edges)
• Inspection of surface preparation
• Inspection before and during application
• Inspection after application

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 pittings
• Surface faults like laminations, etc.

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?

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.

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.

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 conductometric measurement".

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

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.

The sling hygrometre (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.

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.

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.


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

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.

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.

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