In a coating application project, several specifications must be followed. Film thickness measurements are key to ensuring the necessary protection and avoiding defects.
by: Vanesa Restrepo B.
In any coating application project, precision plays a key role from a technical and economic point of view. To the extent that the thickness of the applied film can be controlled, the contractor and the inspector may ensure that the minimum necessary protection is available to the substrate, and that only the amount of paint required to avoid waste of resources is applied.
Numerous solutions are available on the market for measuring the thickness of the coating film, both during application (wet film), and between layers and after the application is completed (dry film).
In the latter case the measurements are required by the inspectors, who must certify the protection (that there will be enough paint to protect the substrate) and the amount (that there will be no cracks or pinholes due to excess layers of paint).
To make this measurement accurate, inspectors use magnetic or ultrasound equipment that determines, without the need for destructive testing, the amount of paint that was applied.
Team Selection
Before purchasing equipment for the measurement of dry film thickness it is necessary to determine the needs of the project. According to Richard Northrop, Chief Marketing Officer of DeFelsko Corporation, the type of equipment will depend on the answer to the following questions:
1. What kind of coatings will you measure? Liquid paints, powder paints, electroplating, anodizing, etc.
2. What are the minimum and maximum thickness ranges of each layer?
3. On which substrates will the measurement be carried out? Ferrous, non-ferrous, wood, concrete, plastic, among others.
4. What is the size and shape of the pieces to be measured?
5. On average, how many prints or screen downloads will you need?
6. How are you measuring those coatings now?
In this sense, Alberto Nicatore, director of Raltec Ltda., Symetry Tecnologica C.A.- BYK Gardner, agrees that the substrate and its curvature are fundamental for the determination of the appropriate type of meter: "The layers involved in the total coating must be clear, be clear if there are associated optical or mechanical variables, need for data processing and storage, actual necessary accuracy and dimensions of the part to be measured".
It is not only an instrument
There are aspects that influence the measurement process and that, when not considered, give rise to failures in the result. That is why Jesús Santafé, operations manager for Venezuela, of the firm ATP Ingeniería recommends verifying that the paint is completely cured and adhered, since exposure to wet elements can affect the operation of meters designed for dry films.
He adds that the calibration of the equipment is vital for the accuracy in the results: "you have to make the corresponding calibrations whenever a measurement is going to be made and check that the paint cured properly, according to the manufacturer's specifications."
This calibration, in the case of magnetic meters, must be done in accordance with the SSPC-PA 2 standard -SSPC Method for measurement of dry paint thickness with magnetic gage-
Alberto Nicatore, meanwhile, refers to some additional aspects that must be taken into account. "The radius of curvature is a very important property because it can contribute to some twisting when measuring, another important factor is the consideration of the complexity and size of the surface, understand that it is essential to determine prior to the measurement what is the number of samples or minimum statistical measurements that must be made on it, so it is essential to have good software".
One of the most common errors in the measurement of film thicknesses comes from the very beginning of the process, when the instrument is not adjusted according to the substrate on which it is going to be worked. Each surface has a different roughness profile, which makes the change of substrate involve a new measurement, according to the manufacturer's manual, to establish what is the zero point of each particular case.
The key is in good use
In the case of coating thickness gauges, as in any other equipment used in the field, it is vital to have an established protocol for care, transport and storage.
In this case, Alberto Nicatore recommends not exposing the equipment to intense magnetic fields such as those of metal detectors and others, avoiding wetting the equipment or exposing it for prolonged periods in environments with high dust or corrosive content and making the measurement only when the surface is completely cured.
Similarly, Richard Northrop points out that once the meter has been put into service, they should always be inspected before use: "Look for obvious damage, especially to the probe or the probe cable. Constant pressure probes should move up and down freely."
The DeFelsko representative points out that while the body of the measuring equipment can work even when scratched, dented or with paint residues, the probe cannot. That is why it is necessary to check the probes and in case of finding dirt, remove it with a soft cloth. In case there are scratches or irreparable defects, it is necessary to replace the part so that the result is accurate.
Likewise, you have to be careful with hot or rough surfaces, in case there is a high temperature, the equipment should be left to rest between one measurement and another. In case the substrate is rough, then it is recommended not to drag the meters and their parts to the sides, as that would generate scratches.
Inspectors can tell that the probe needs to be changed when the meters make readings with values much lower than expected. When the readings, on the other hand, are much higher, it is likely that there are traces of paint adhered to the surface of the sensor.
Once the measurement process has been completed, the instrument must be placed back in its bag or in the original packaging, with the test tip covered. It is advisable to store it in a cool and dry place, away from high temperatures or magnetic fields. And as a practical tip, it is recommended to always carry spare batteries, to avoid unwanted delays in field work.
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