RELIABLE METHODS FOR THE MEASUREMENT AND INSPECTION OF PROTECTIVE BARRIER COATINGS FOR OUTDOOR MONUMENTS

By Joseph Sembrat

Abstract
Equipment and techniques currently used in coating applications and inspections, which are standard in the protective coatings industry, are not utilized by the conservator of outdoor monuments. Premature coatings failure is responsible for millions of dollars of damage each year, and aggressive efforts to combat this problem by organizations such as the National Association of Corrosion Engineers (NACE) and the Society for Protective Coatings (SSPC) are evident by the vast amounts of research and technical articles which are being published annually on the subject.

The present practice in use by conservators of measuring coating thickness by the number of coats applied is far too inaccurate. Additionally, many of the coatings they apply are highly susceptible to premature failure because present application and inspection techniques do not include the testing of the film’s continuity.

The test equipment industry manufactures film thickness gages for measuring the proper thickness of applied coatings and holiday detectors which test a coating’s continuity. The integrity and longevity of protective coatings on outdoor monuments could greatly increase if these instruments were utilized by conservators during application and inspection processes.

Introduction
Because of their ability to control corrosion on metals, protective coatings have been applied to monuments as far back as to the ancient Greeks. Different forms of waxes and oils were applied to bronze objects by Greek craftsmen to help protect this noble metal and to also add to the aesthetic qualities of the highly finished surfaces. After the Industrial Revolutions in Europe and the United States, paints were used in the arts and architecture either to conceal inferior materials or for their aesthetic properties.

It is interesting to note that many of the same coating application techniques used throughout history are still being employed today in the conservation of outdoor monuments with varying degrees of success. This does not imply that the techniques of the past are inferior to those used today, rather it implies that when the techniques of the past are employed using modern materials, the end results are not always favorable. To illustrate this point further, one need only look at the use of lead-based paints and primers. It is generally agreed that these products outperform even our best coatings available today. The reason is not because the products were applied better than they are today, instead the coating was more “forgiving” when imprecisely applied.

The very fact that our modern coatings require more precise tolerances than those of the past should alert us that we need better techniques to monitor how these materials are applied and inspected. It would be a great improvement to the practice of outdoor monuments conservation if we followed the manufacturers’ specifications which clearly explain how thick a coating must be in order for it to perform properly, rather than follow the present practice which dictates how many coats are to be applied. The coating industry recognized this several decades ago and instituted numerous guidelines and procedures for the proper application and monitoring of coatings throughout the world.

The author has been using a coating thickness gage and holiday detector for approximately three years with increasing degrees of success in the initial application of protective barrier coatings and their subsequent monitoring and maintenance. Conservation Technical Associates LLC has saved thousands of dollars by preventing costly expenses associated with premature coating failures. The use of a film thickness gage has helped eliminate the application of excess quantities of coating materials while the holiday detector has enabled us to pinpoint flaws and correct them before the treatments were complete.

This paper will define the techniques and equipment used by coating industry applicators and inspectors and discuss how the conservator of outdoor monuments can utilize them to achieve coatings of the highest quality.

Film Thickness Gages
Used extensively by applicators and inspectors in the coatings industry field, film thickness gages are indispensable tools which measure the thickness of applied coatings. The proper thickness of any protective film is critical as it directly effects the performance of the coating. All coatings manufacturers specify the proper wet film thickness (WFT) and/or dry film thickness (DFT) that is optimal for their products. These film thicknesses are usually expressed in mils in the US (1 mil = 0.001 inch = 0.0254001mm = 25.4001 microns) and microns where the SI system of units is used. The WFT is the measurement of a coating above the substrate while still wet, whereas, the DFT is the measurement of the coating above the substrate after it has dried. Film measurements outside of the manufacturers’ specifications could negate product warranties or guarantees.

To better familiarize themselves with the methodology and terminology used in the application and inspection of coatings, it is recommended that the conservator obtain copies of the following American Society for Testing and Materials (ASTM) or international equivalent standards: ASTM D1212 “Measurement of Wet Film Thickness of Paint, Varnish, Lacquer, and Related Products”; ASTM D4414 “Practice for Measurement of Wet Film Thickness of Organic Coatings by Notch Test”; ASTM D1005 “Measurement of Dry Film Thickness of Organic Coatings”; ASTM D1186 “Measurement of Dry Film Thickness on Non-Magnetic Coatings Applied to a Ferrous Base”; and ASTM D1400-81 “Measurement of Dry Film Thickness of Nonmetallic Coatings of Paint, Varnish, Lacquers on a Nonmetallic Base”.

There are essentially two categories of film thickness gages, WFT gages and DFT gages.

Wet Film Thickness Gages
The WFT gage of choice, and probably most useful to the conservator, is the notched-type (See Figure 1). Although it is available in various shapes and styles, the operating principle is the same. While the coating is still wet, the gage is placed on the film at a 90? angle to the surface. The gage is then pressed into the film until it rests firmly on the substrate. When removed, it is noted which one of the deepest teeth has the coating on it and which one of the next higher teeth does not. The WFT will fall between these two readings. The disadvantages of this procedure is that unless the coating is able to “level” itself out, this technique is usually “destructive” to that area. If it is not possible to perform this test directly on the surface to be coated, an alternative means of estimating the WFT, using the notched gage, is to first work out the proper application parameters on a test surface, and then use these same techniques on the substrate to be coated. Although not as accurate a process, it is more accurate than guessing if the thickness is correct.

WFT and DFT are directly related and therefore usually specified by the coating manufacturer. Additionally, because the coating is being measured while it is still wet, the operator will be able to apply more material if the film is too thin, or it will alert him that the application is too heavy if the film is too thick.

Dry Film Thickness Gages
DFT gages are numerous and sometimes more complicated in their operation than WFT gages. There are three general categories of DFT gages; destructive, non-destructive mechanical, and non-destructive electronic.

Destructive DFT gages, as the name implies, require “destructive” approaches for measuring film thicknesses but are sometimes more versatile because they can be used for both wet and dry testing and are capable of measuring the thickness of metallic coatings on non-magnetic substrates (See Figure 2). The principle behind the operation of these gages is that they measure the difference in height between the surface of the coating and an exposed area of the substrate. This is accomplished by carefully scraping off a 3mm gap in the coating down to the substrate. The two outer legs on the gage are placed on either side of the gap so that the center plunger, which moves up and down, is allowed to come into contact with the substrate. The coating thickness is then read off of the dial indicator. Although suitable for measuring magnetic and non-magnetic coatings on both magnetic and non-magnetic substrates, the destructive thickness gage would probably have limited applications in the conservation of outdoor monuments as its use is detrimental to the coating.

Non-destructive mechanical DFT gages (See Figure 3) are typically the least expensive ($200 to $300) and most compact of all the gages but are only useful for measuring the thickness of non-magnetic coatings on magnetic substrates. These units consist of a special lightweight magnet attached to a spring which is usually contained within a pencil-like tube. Measurements are taken by placing the magnetic tip onto the coated surface. The operator then pulls the body of the gage slowly away from the surface, thus extending the spring. The spring extension is directly proportional to the thickness of the coating which is easily obtained by reading the graduated scale on the side of the tube. It is highly recommended that a gage be purchased which allows overhead and vertical readings as well as horizontal ones. This affordable and highly accurate DFT gage is ideal for conservators who have a limited budget and perform treatments mainly on ferrous objects.

Non-destructive electronic DFT gages are the most versatile but are considerably more expensive ($800 to $2,000) than the destructive and non-destructive mechanical gages. In addition to measuring DFT of coatings on ferrous substrates, some of the electronic units are capable of measuring films on ferrous and nonferrous metals and other materials such as wood, concrete, plastic, glass, and ceramics. Several of the units use electromagnetic and/or eddy current principles while others utilize ultrasonic technology. The more advanced units offer additional features like the ability to measure substrate thickness, and others are equipped with a built-in memory and serial port which allows for the reading, storage, and downloading into a personal computer of over 10,000 measurements.

The non-destructive electromagnetic gages are used to measure DFT of non-magnetic coatings on ferrous substrates. The instrument probe uses direct current (DC) to induce a magnetic field which then interacts with the ferrous metal substrate in exactly the same manner as the non-destructive mechanical DFT gage described above. The strength of the substrate’s influence is sensed within the probe, and the resulting electrical measurement is converted to a thickness reading on the screen or dial of the instrument. Although generally very reliable, there are factors which negatively influence the performance of the electromagnetic DFT gage if it can not first be zeroed on an uncoated portion of the substrate. Some of the factors are: the magnetic properties of the substrate, substrate thickness, proximity to edges, curves, magnetic coatings, and the configuration of the substrate.

Instruments employing eddy current principles are used to measure the DFT of non-conductive films applied to conductive substrates, most often to such nonferrous metals as aluminum, copper, brass, and stainless steel. The instrument may look exactly like the non-destructive electromagnetic gage, but it works by inducing an eddy current in the substrate by means of alternating current (AC) fed to the probe. Although measurements can be made on any conductive metal, the effects of the shape and size of the probe and the conductivity of the metallic substrate are significant. Factors which affect the accuracy of the eddy current gage if it cannot first be zeroed on an uncoated portion of the substrate are: magnetic and conductive properties of the substrate, substrate thickness, edges, curves, conductivity of the coating.

Instruments capable of operating under both electromagnetic and eddy current principles usually use a separate probe for each principle but some manufacturers have developed probes that can perform both functions (See Figure 4). Eddy current probes are more specialized than electromagnetic probes and are generally used over a smaller range of DFT. Depending upon the materials and specific needs of the conservator, a DFT gage with both capabilities should prove sufficient for most DFT readings while still being affordable.

The author has encountered difficulties in obtaining consistent and reliable readings from rough or uneven surfaces with some of the more basic models of non-destructive electronic gages. It seems that porous cast surfaces or heavily textured surfaces cause these gages to fluctuate between plus and minus readings even when no coatings are present. To help combat this problem, some manufactures offer options for the gage which will automatically calculate the mean of a set of average readings. This feature is extremely useful as it will automatically eliminate erroneous readings and only average the most consistent information. This option is highly recommended for the conservator who works on heavily weathered outdoor monuments as it is really the only reliable way to achieve accurate DFT readings.

Non-destructive electronic ultrasonic DFT gages (See Figure 5) can accurately measure coatings on most types of metal, wood, plastic, glass, and ceramic and are also capable of measuring substrate thickness depending on its composition and thickness. Only slightly larger than the electromagnetic and eddy current gages, the ultrasonic instrument is still very compact and durable. The gage works by an ultrasonic pulse which is transmitted through the coating to the substrate. It is then reflected back to the transducer and is converted into a high frequency electrical signal. The echo wave form is then digitized and analyzed to determine the coating thickness. A couplant, such as water, is typically used to assist in the transfer of the ultrasonic pulse from the probe to the coating.

The ultrasonic instrument is very accurate (? 2 microns) and is capable of storing over 1500 readings in its memory. Since the gage is equipped with a RS232 data output, the readings are easily downloaded into a personal computer. An ultrasonic gage is an ideal tool for conservators who work on a wide variety of substrates, and although its initial expense of $3,000 to $4,000 is relatively high when compared to the other gages, its versatility may help to defray the cost.

Holiday Detectors
Pinholes, voids, and misses which cause discontinuity in a coating are known as holidays. No matter how small some of these defects may be, it is possible for moisture and corrosive materials to pass through them and to come into contact with the underlying substrate. Such defects are usually the cause for expensive, and sometimes damaging, premature coating failure. The holiday detector allows the conservator to identify these defects in non-conductive coatings and correct them either before the object is to be placed outside or during scheduled maintenance inspections. Early detection and repair of holidays can reduce the need for costly retreatments as they will greatly extend the overall life of the coating.

An electrical holiday detector is a device which locates areas or points on a non-metallically coated metal surface where there is a great difference in the electrical resistance between an exploring electrode and the underlying conductive substrate. A typical holiday detector consists of an electrical energy source such as a battery or high voltage coil, an exploring electrode, and a connection point from the energy source to an uncoated area on the conductive substrate. Usually some form of alerting device such as a bell or light is used to alert the operator that a holiday has been detected. Because most protective coatings used in the conservation of outdoor monuments are good electrical insulators, this device has proven itself to be invaluable during the application and inspection phases of numerous conservation treatments. The holiday inspection should be done in accordance with either the guidelines set forth by NACE in its Standard Recommended Practice RP0188 “Discontinuity (Holiday) Testing of Protective Coatings” or ASTM D 5162-91 “Standard Practice for Discontinuity (Holiday) Testing of Non-conductive Protective Coating on Metallic Substrates”.

Currently there are two types of holiday detectors in use. Thin films (less than 508 microns or 20 mils) are usually tested using a low voltage, wetted-sponge type. Films applied at thicknesses in excess of 508 microns require a high voltage spark type detector. The sponge-type detector (See Figure 6) is considered “nondestructive” because its use does not alter the integrity of the coating. Since most coatings applied by the conservator of outdoor monuments will be less than 508 microns thick, the sponge-type detector should prove to be most useful.

The sponge-type system operates at approximately 75 volts or less. It works by applying current to the coating with an electrode consisting of a cellulose sponge dampened with an electrically conductive liquid such as a surfactant diluted with tap water. The electrode is rubbed over the coated surface and when a holiday is encountered, current travels through the moisture in the sponge to the metal substrate and back to the device. This movement of current completes the electrical loop and activates the signaling light and/or bell. The conservator would then mark the area with a piece of tape or a removable marker and continue until the entire surface has been tested.

These areas of discontinuity would then be recoated and tested a final time to confirm that they have been properly repaired.

The spark-type detector operates with voltages ranging from 1,000 to 30,000 volts. A wire brush or rolling spring electrode is used in a manner similar to the sponge-type system, but because of the high voltage, no electrolyte is necessary. If a holiday or very thin area of the coating is passed over with the electrode, a spark will arc from the electrode to the metal substrate, thereby activating the signal device. The technique is considered “destructive” because the excessively high voltage will stress some of the sound coating around the holiday and require its removal before the area can be recoated.

Conclusion
The use of WFT and DFT gages during the application and inspection of a protective coating on outdoor monuments is critical as it: 1) insures that the proper thickness of the coating has been applied over the entire monument; 2) permits the conservator to monitor the deterioration of the coating over a period of time; and 3) reduces the overall cost of the treatment, and subsequent maintenance of the monument, by providing the conservator with quantitative figures which prevents excess application of coating materials.

The use of a holiday detector during coating inspections is the most accurate and rapid way of determining a coating’s continuity. Its use will pinpoint flaws in a coating and insure that they have been properly repaired.

When used in conjunction with each other, these instruments will help conservators achieve uniform and continuous coatings. This process should also assist in the creation of a much-needed protocol for the proper application of coatings for outdoor monuments.

Acknowledgments
The author would like to thank the following individuals and companies for their invaluable assistance: Chris Blum, Linda Merk-Gould, Francis Miller, and Patty Miller of Conservation Technical Associates LLC; David Beamish of DeFelsko Corporation; Paul N. Gardner of Paul N. Gardner Company, Inc.; Martin Weaver of the Center for Preservation Research at Columbia University, and Julya Sembrat for her excellent editing work.

Materials
Dry Film Thickness Gage
- Positector® 6000 Series, Positector? 100, and Posipen™. DeFelsko Corporation, 802 Proctor Avenue, P. O. Box 676 Ogdensburg, NY 13669-0676, USA.
- Model 126 Dial Thickness Gage. Paul N. Gardner Company, Inc., 316 N.E. First Street, Pampano Beach, FL 33060, USA.

Wet Film Thickness Gage
- GARDCO “Calling Card”. Paul N. Gardner Company, Inc., 316 N.E. First Street, Pampano Beach, FL 33060, USA.

Holiday Detectors
- M/1 Holiday Detector. Tinker and Rasor, 417 Agostino Road, P.O.
Box 281, San Gabriel, CA 91778-0281, USA

Standards
- American Society for Testing and Materials (ASTM). 100 Barr
Harbor Drive, West Conshohocken, PA 19428-2959, USA. Web-site:
http://www.astm.org.

- National Association of Corrosion Engineers (NACE). P.O. Box
218340, Houston, TX 77218-8340, USA. Web-site:
http://www.nace.org.

- Society for Protective Coatings (SSPC). 40 24th Street, Pittsburgh, PA
15222, USA. Web-site: http://www.sspc.org.

Résumé

Les équipements et les techniques utilisés pour l’application des revêtements et pour leur contrôle, qui sont couramment utilisés dans l’industrie des revêtements protecteur, ne sont pas en usage par les conservateurs-restaurateurs de monuments en extérieur. C’est en millions de dollars que l’on peut estimer les dégâts causés par les mauvais revêtements, et des efforts considérables sont mis en oeuvre par des organisations telles que National Association of Corrosion Engineers (NACE) et the Society fo Protective Coatings (SSPC). La quantité d’articles techniques publiés chaque année montre bien l’ampleur de ces recherches.

La pratique actuelle pour la mesure des épaisseurs de film par les conservateurs-restaurateurs se limite à dénombrer les couches de revêtements appliqués, ce qui est vraiment imprécis. De la même manière, la plus part des revêtements utilisés sont susceptibles de se dégrader en raison des méthodes d’application et de l’absence de contrôle de la bonne continuité du film.

Parmi les équipements de contrôle commercialisés, on peut noter des jauges pour la mesure de l’épaisseur des films appliqués, et des détecteurs testant la continuité des films. L’intégrité et la longévité des revêtements protecteurs sur les monuments en extérieur pourraient être considérablement améliorés si les conservateurs-restaurateurs les utilisaient durant l’application et pour les contrôles.