The control of film thickness when applying BelzonaÒ coatings is important to ensure that the minimum thicknesses required for satisfactory performance are achieved. Where technically possible the BelzonaÒ coatings (e.g. BelzonaÒ 1321 & BelzonaÒ 1341) are recommended to be applied as multi-coat systems, which assists in obtaining uniform coverage with minimal risk of un-protected substrate.

Recent coating developments have resulted in more densely cross-linked systems less tolerant to overcoating and which have necessitated the application of high-build single coat systems such as BelzonaÒ 1391 & 1591.

To assist in the correct application of these systems, we supply wet film thickness gauges, which should be regularly used during application to ensure the correct film thickness is being applied.

In addition, the practical coverage rates quoted should be accurately followed, as this also will ensure the minimum thicknesses required are being achieved. Ongoing visual inspection during the application of these one-coat systems is also vital to ensure minimal defects in the system.

The most common instrument for measuring wet film thickness is the Wet Film Thickness Comb. These are available in a wide range of sizes, designs and materials. Both Stainless steel and disposable plastic types are available. They all consist of teeth of various lengths cut into their side. The comb is dipped into the applied coating immediately after application, removed and examined. The wet film thickness lies between the shortest wet tooth and longest dry tooth values. These gauges are used in line with various International standards including ASTM D 4414-A and ISO 2808-7B.

2. Dry film thickness

Dry film thickness measurement equipment uses Magnetic, Eddy Currents or Electromagnetic induction to detect the thickness of coatings applied onto metal substrates.

These measure the thickness of non-magnetic coatings on carbon steel, cast iron, duplex and ferritic stainless steel substrates. Requiring no batteries these instruments with an accuracy between 5-15% are commonly used in factories, workshops and on site for general quality control. There are various forms of these gauges and following are 3 of the more common.

    a) "Banana gauge"

The gauge is held against the test surface and knurled wheel turned so that the magnet makes contact with the surface. The wheel is then turned in the opposite direction until the magnet is pulled off the surface, an indicator pops up and an audible click is heard. The film thickness is indicated on the wheel.

    b) Pull off gauge

This is a pen-shaped comprising a magnet attached to sensitive spring. The magnet tip is placed on the coating surface and the gauge gently raised. When the magnet "breaks" from the surface the reading viewed at this point indicates the film thickness.

    c) Elcometer 101

One of the oldest portable film thickness gauges. The 2 chromed spherical contacts are placed on the coating surface. The dial displays the coating thickness.

For measurement on ferrous substrates Electromagnetic induction is used whilst on non-ferrous metal substrates eddy currents are used. Separate or combined instruments, with different probes, are available to measure on both types of surface. The accuracy of these instruments is typically ±1%. They also allow for recording and statistical analysis of the results. They are simple to use, the probe being held on the surface and the thickness displayed on the instrument. The accuracy is increased by calibration of the instrument using shims of the coating thickness over the prepared substrate.

England
Elcometer Instruments Ltd
Edge Lane
Droylsden
Manchester
M43 6BU
Tel (0161) 3707611
Fax (0161) 3704999

Paint Test Equipment
3/4 The Courtyard
Greenfield Farm Estate
Congleton
Cheshire
CW 12 4TR
Tel (01260) 275614
Fax (01260) 299231

USA
Elcometer Inc
1893 Rochester Industrial Drive
Rochester Hills
Michigan 48309
Tel (810) 6500500
Fax (810) 6500501

Germany
Elcometer Instruments GmbH
Albblick 18
D-73730 Esslingen
Tel (0711) 366081
Fax (0711) 366455