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The prevailing climatic conditions during blasting and painting are critical in achieving a successful coating application.

Dewpoint
In certain conditions moisture from the air can condense on the substrate post blast resulting in flash rust which can adversely affect the adherence of subsequently applied paint. Furthermore moisture can condense on the substrate after the application of the paint before the curing process is complete.

In order to monitor the dewpoint temperature the following parameters must be measured: the surface temperature of the substrate, the air temperature, and the relative humidity. By applying a formula to the air temperature and the relative humidity the dewpoint can be calculated – the surface temperature of the substrate must remain at least 3 degrees Celcius above the dewpoint temperature to avoid moisture condensing on the surface.

Temperature
The temperature must always be monitored to ensure that it remains within the parameters of the coating specifications. The curing process can fail if the temperature is too low or too high.

DTH100 Digital Thermometer – high res

DPM120 Dewpoint Meter – high res

MTH Magnetic Thermometers – high res

WHM130 Whirling Hygrometer – high res

High performance paint systems normally require that the steel surface is blasted not only to achieve cleanliness but also to achieve a certain profile which allows the coating to adhere correctly. The depth of the profile achieved in the steel can be altered by the use of different types and sizes of abrasives blasted at different pressures onto the surface. Therefore the blasting process has the result not only of cleaning the steel to a recognized standard but also of creating a profile to which the coating can adhere.

If the profile is too shallow the coating system will not adhere correctly potentially resulting in premature failure, if the profile is too deep the amount of coating required increases and the potential for multi-coat systems to adhere incorrectly increases – furthermore if the profile is much too deep the tops of the peaks in the profile may be exposed leading to rust spots.

SPG360 Surface Profile Gauge – high res

SRC340 Surface Roughness Comparator – high res

TXT300 TXG320 Testex Surface Roughness Tape and Gauge – high res

High performance coating systems require that each application is of a specified thickness when dry. The thickness of the coating is normally measured in microns (one micron is one thousandth of a millimeter).

Wet film thickness
In order to assess the thickness of the coating whilst the application is ongoing a wet film thickness gauge can be used – the wet film thickness reading can be used as a guide to estimate the dry film thickness.

Dry film thickness
Once the coating is dry there are two methods of measuring the dry film thickness: destructive and non-destructive. The most popular method is non-destructive using a dry film thickness gauge. Dry film thickness gauges use physical properties to accurately measure the thickness of the coating quickly and easily in locations chosen by the operator.

Different gauges have different performance capabilities, some basic gauges are used simply to check the film thickness and cannot store or analyse readings, more sophisticated gauges can record multiple readings, the information can be stored in the gauge, and downloaded to a computer for analysis and long term storage.

WFT-450/455 Wet Film Thickness Gauge

DFT-520 Dry Film Thickness Gauge

DFT-540 Series Dry Film Thickness Gauges

Assuming that the initial steps in the blasting and painting process have been carried out correctly the substrate should have been blast cleaned to provide the required standard of cleanliness and the correct surface profile, and the coating should have been applied at the correct film thickness, with both processes taking place in climatic conditions which avoid unwanted condensation on the substrate or coated surface.

The next step in the process is to verify that the coating is adhered to the substrate correctly – if the coating is not adhered correctly the coating may suffer premature failure.

There are two basic types of adhesion test: destructive and non-destructive. As well as providing immediate results to satisfy job completion procedures, the non-destructive adhesion test can be used as part of a long term maintenance programme.

CHC520 Cross Cut Adhesion Test – high res

HAT500 Hydraulic Adhesion Tester – high res

The coating applied to the substrate should protect against premature corrosion. The integrity of the coating can be assessed with respect to porosity and remedial work carried out if required. Porosity in the coating can be caused by numerous abnormalities in the coating such as:

• Pinholes in the coating caused by air escaping from the surface of the substrate through the coating during the curing process.
• Uncoated areas especially prevalent on corners or weld seems where coatings may have flowed away leaving the peaks of the profile exposed.
• Cracks in the coating caused by excessive wet film build creating pressure during the flow stage of the curing process.

HOD600 Holiday Detector – high res

PHD620 Pin Hole Detector – high res