Wet film is the coating on a surface which has not yet dried or cured. Measuring wet film thickness is important for financial, logistical, aesthetic and quality reasons. The prime benefit is that the operator applying the coating can make an informed decision on the quality of a coating application, enabling any remedial work to be carried out immediately.
A significant proportion of the total cost of re-painting a structure is gaining access and hiring the equipment such as hoists and scaffolding. Given that the costs for this equipment would have to be incurred again if the job is not done correctly the first time, it is more cost effective to check the thickness of the paint whilst it is wet, in order to make any necessary adjustments.
Applying too little paint will reduce the protection of the substrate, possibly exposing profile peaks and causing instant corrosion known as rust rash. Even if the profile peaks are covered by a coating but the coating is too thin, the protection offered will still be reduced. This will happen particularly in service applications, such as ship hulls, pipelines and internal coatings, where the coating can become eroded during its life time, resulting in either premature failure or reduced wear resistance.
In addition, if the paint is applied too thinly the surface will not be covered sufficiently and the original surface could be visible through it.
If the paint is applied too thickly, more paint than necessary is being used, thus increasing cost. Applying too much paint can also cause a cured ‘skin’ to form on top of the coating whilst the paint below the surface is left uncured. This reduces adhesion and increases the likelihood of the skin cracking which can lead to moisture ingress and subsequently; premature coating failure.
By measuring the thickness of the wet film, an early indication of the quality of the coating application can be given, enabling a change in application to be made, if needed, at the most convenient and cost effective time.
One of the main methods of measuring wet film thickness in the coatings inspection field is by using wet film combs.
A wet film comb is either hexagonal with a series of teeth along each edge or rectangular, which has teeth on the long edges or the short edges only but not both. The hexagonal combs have the advantage of having 6 different scales on one comb, whilst the rectangular combs have more teeth along the long edge, resulting in greater accuracy due to improved resolution.
Occasionally referred to as ‘notch gauges’, wet film combs are available in plastic, aluminium and stainless steel with various measurement scales. Plastic combs are low cost and can be kept as a permanent record, but can only be used once so the relative cost of using these combs may be irrelevant over time. Alternatively, there are reusable options such as aluminium combs, which are more accurate than the plastic ones. The stainless steel combs, however, are the hardest wearing and most accurate of the three.
The scale range available from Elcometer is 20 to 3,000µm, enabling the measurement of a range of coatings such as paint on metal substrates through to fibreglass layers and road markings. The chosen scale should encompass the required or expected wet film thickness.
Each scale consists of a number of teeth, each shorter than the previous one. The height of the teeth above the substrate increases by a fixed amount with each subsequent tooth.
The comb is held vertically and placed into the wet paint with the outer teeth of the chosen scale making contact with the substrate at a 90°angle. It is held in position for a few seconds, to ensure that any teeth which contact the paint are coated and then removed from the film and examined.
The teeth that are long enough to reach the paint will be coated at the end and those that remain above the paint will not have any coating on them. Depending on the standard which is being worked towards, the wet film thickness of the coating is either the last coated tooth, or between the last tooth that’s coated and the first tooth that is dry.
Take a look at Elcometer’s full range of Wet Film Combs here.
For a step-by-step tutorial video on how to use wet film combs, click here.