Why is Wet Film and Dry Film Paint Measuring Crucial?


  • Author Andy Potts
  • Published October 27, 2020
  • Word count 1,466

The lowly wet film comb is a little used object in the paint world, but for 50p this simple device can save you thousands in wasted paint materials and hours of labour refurbishing projects - why? Explained below.

Wet Film Combs

Stainless Steel WFT Comb

The Stainless Steel Wet Film Thickness Gauge is a hexagonal or rectangular precision measuring comb made of heavy stainless steel. The high-grade stainless steel will not be affected by acid or base elements and are long lasting, reusable and supplied in a range of thicknesses measuring up to 10,000 μm (120 mils).

Wet Film Thickness Gauges are designed to measure the thickness of coatings immediately after they have been applied to a substrate. The gauge has a series of notches cut into its sides like the teeth on a comb. The wet film thickness of coatings including paints, resins, lacquers, varnishes and gels etc can be measured.

Using a Wet Film Comb

Before you start, ensure your Wet Film Comb is clean and undamaged :

Place the gauge at a 90° angle to the coated substrate Make sure the gauge is in firm contact with the substrate. Remove the gauge in a straight line up. Assess the teeth of the gauge. The wet film thickness lies between the largest tooth that is wet and the smallest tooth that is not wet. To use the comb on pipes, ensure it is placed parallel to the longitudinal axis of the pipe. On rough surfaces, measurements will be made from the surface peaks and therefore will represent the minimum wet film thickness.

Aluminium WFT Comb

The punched aluminium combs offer the user a low cost method of measuring the wet film thickness. However, the aluminium combs are not as accurate as precision formed stainless steel wet film combs and has a shorter lifespan.

Plastic WFT Comb

The Plastic Wet Film Combs are made from ABS plastic and are designed to be used once and kept as a record of wet film thickness measurement for quality assurance or customer requirements. Metric and Imperial values are on the same wet film comb, 50 to 800µm on one side, 2 to 32 mils on the other.

As the plastic material is susceptible to chemical & base elements, these wet film combs are strictly for one time use and should be either disposed off or kept as a record after use.

Wet Film Wheels

The Wet Film Wheel is a high precision, accurate and easy to use instrument which consists of a set of three wheels. The central wheel is of a smaller diameter and is eccentric relative to the two outer wheels.

By rolling the gauge through a wet coating, the centre wheel eventually touches the film. This point on the scale indicates the thickness. When the volume to solids ratio of the coating is known (generally found on the product data sheet supplied by the manufacturer), the wet film thickness can be used to predict the dry film thickness.

Several measurement ranges between 0 to 25µm and 0 to 1000µm (0 to 1 mil and 0 to 40 mils) are available. The wet film wheel is suitable for both flat and curved surfaces.

So you've successfully measured your paint but what about your dry paint?

Dry Film Thickness Measurement

A dry film thickness gauge is an instrument used to measure the coating thickness on either a metal or non-metal substrate.

Dry Film Thickness (DFT) is an important variable that plays a role in product quality, process control and cost control. Coatings are designed to perform at optimum performance when applied within the specified range as stated by the manufacturer.

DFT measurements can be used to evaluate a coating’s expected life, the product’s appearance & performance and ensure compliance with contract specifications or standards.

Dry film thickness can be measured using two methods :

  1. Non-Destructive Thickness Measurement

‣ Methods which do not damage the coating or the substrate such as magnetic, magnetic induction & eddy current.

  1. Destructive Thickness Measurement

‣ Methods which involve cutting the coating to the substrate using a special cutting tool.

Mechanical Coating Thickness Gauge

The Mechanical Coating Thickness Gauge, commonly referred to as the “Banana Gauge”, is a Type I instrument designed for non-destructive measurement of non-magnetic coatings (paint, enamel, galvanizing, plating, etc) on steel.

It is suitable for use in high risk areas such as high temperature or flammable atmospheres, underwater or where the risk of explosion is high and the use of electronic instruments is generally prohibited.

Digital Coating Thickness Gauge

Digital Coating Thickness Gauges use magnetic and eddy current principles to measure coating thickness on both ferrous and non-ferrous metals, accurately and quickly.

DFT measurements can be carried out on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium with the digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.

Coating Thickness Gauge Calibration

Formal quality systems, such as those described in ISO 9001, require coating thickness gauges to be properly controlled, logged and calibrated. In order to test the accuracy of a particular gauge, it is important to have traceable coating thickness standards.

Calibration foils or shims are the most convenient way of creating a coating thickness standard on the substrate material, surface finish or form. Shims provide the ideal method for adjusting the calibration of the coating thickness gauge to ensure the greatest possible accuracy.

Coating Thickness Standards

As mentioned above, most organisations require verification of gauge accuracy at the test site each time a coating thickness gauge is put into service and at frequent intervals during use. There are three types of coating thickness standards commonly available :

Certified Coated Metal Plates

Certified Polystyrene Blocks

Certified Plastic Shims

Certified coating thickness standards should have measured values which are traceable to a National Metrology Institution such as NIST or PTB and individually serialised for the traceability.

Certified coated metal plates & polystyrene blocks are ideal for verifying the accuracy & operation of coating thickness gauges and are an important component in fulfilling both ISO & in-house quality control requirements. They can be used to verify the accuracy & operation of any Type 1 (mechanical) and Type 2 (electronic) magnetic, eddy-current or ultrasonic coating thickness gauge.

Plastic Shims

Certified Plastic Shims (foils) provide an economical & convenient alternative to coated metal plates, though they have a reduced accuracy of ±2 µm (±0.08 mil). A coating thickness gauge measures shim thickness when the shim is placed over a smooth metal surface (i.e. zero plate). The certificate of calibration showing the traceability should be included with each shim or set of shims.

Zero Plates

Zero Plates are ideal to test a coating thickness gauge’s functionality & calibration and when it may be difficult or impractical to obtain an uncoated substrate.

Coated Metal Standards for magnetic & eddy current gauges usually include an uncoated zero plate. The first gauge measurement is taken on the zero plate to verify that the gauge measures zero correctly. If not, digital gauges can usually be adjusted to "0" on the zero plate. After this adjustment is performed, the gauge should measure within tolerance on the coated standards.

It also provides a convenient base onto which plastic shims & polystyrene blocks can be placed for verification purposes.

Check zero, adjust to "0" if necessary, then place the shim/block on the zero plate and measure.

Ultrasonic Coating Thickness Gauge

Ultrasonic Coating Thickness Gauge measures coating thickness non-destructively over wood, concrete, plastic, etc using ultrasound technology. To help the ultrasonic signal penetrate the coating, a liquid couplant is applied to the measurement area. The ultrasonic couplant (gel) is ideal for rough coatings and for vertical surfaces. Alternatively, a drop of water is often sufficient for smooth coatings on horizontal surfaces.

Place the probe on the coating and press down to measure. The instrument sends a sonic pulse into the coating and listens for a reflection from the coating-substrate interface, displaying the thickness measurement on the display.

Destructive Coating Thickness Gauges

Paint Inspection Gauge (P.I.G.) / Tooke Gauge

A Paint Inspection Gauge is a testing tool used to determine the overall thickness, or the individual layer thickness, of a dry paint or coating. A small incision is made through the coating down to the substrate and the V-shaped groove is then observed through a microscope on the gauge where a graduated scale is used to measure the thicknesses of the coating layers, provided they have different colours. The width of the incision is then related to the coating thickness. Please take note that the test is destructive to the coating and therefore, it should be used with caution.

So now you know how to take a wet film and dry film thickness reading for industrial paint applications. If you have any further questions please use the Bio for more info.

Author Andy Potts | Created 15th October 2020 | Andy is an author and specialist in business, paint, automotive, flooring and property. Working for https://www.ultrimaxstore.com/ a leading industrial paint shop specialist with a full range of Jotun Paints https://www.ultrimaxstore.com/jotun-b-6.

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