Overspray and Dry Fall Coatings
Dry fall coatings (also known as dry fog coatings) are spray-applied protective and/or decorative paints specifically designed to minimize attachment of any overspray to any nearby areas and equipment that are not desired to be painted. Overspray consists of the tiny droplets of paint created during the spraying process that do not deposit where intended but wander through the air and eventually deposit onto other areas, such as equipment, buildings, or vehicles. These droplets, if still wet when deposited, can subsequently dry and tenaciously attach to these unintended objects. If spraying indoors, flooring and any equipment on the floor below can be contaminated with this overspray. If spraying outdoors, any property or equipment either below and/or downwind from the application area may be contaminated. Dry fall coatings are formulated so that when sprayed within the spray equipment and environmental parameters recommended by the manufacturer, any overspray dries within a certain distance before depositing onto these other objects and does not aggressively attach to them. Typically, the dried overspray can then easily be wiped, brushed, swept, vacuumed, or washed off as a dry powder and disposed of.
Indoor vs. Outdoor Application
Dry fall coatings can be applied both indoors and outdoors. Since dry fall coatings are spray-applied, their use can save quite a bit of time when compared to painting by using rollers or brushes. A typical indoor use is for painting ceilings and overhead support structures, either for new building construction (direct-to-metal) or for refurbishment of previously painted (and properly prepared) ceilings in industrial, commercial, and institutional buildings like offices, warehouses, stores, hotels, hangars, and manufacturing plants. Outdoor uses include the painting of tanks, towers, piping, equipment cabinets, and ceilings in parking garages.
Water-Based vs. Solvent-Based Coatings
Dry fall coatings can either be water-based or solvent-based. Water-based dry fall coatings are typically made using acrylic polymers and are used indoors. They mostly provide improved aesthetics (color and gloss) and lighting efficiency (white or lighter colors only), with some protection from weathering and corrosion. Water-based dry fall coatings for exterior use do exist, but typically do not provide the amount of weathering, corrosion, and mold and mildew protection that solvent-based dry fall coatings can. Solvent-based dry fall coatings can use acrylic, alkyd, epoxy, and/or urethane polymers, depending on the degree of weathering and corrosion resistance required. They can even be compounded with zinc additives to provide greater corrosion resistance. Although solvent-based dry fall coatings for interior use do exist, due to their flammability one must ensure there are absolutely no sources of potential ignition of solvent-vapors present in the enclosed area when using them.
Surface Preparation and Application
It is vitally important to consult the manufacturer’s Product Data Sheet (PDS)/Technical Data Sheet (TDS) and Safety Data Sheet (SDS) for the specific dry fall coating to be used. The PDS will provide guidance on the surface preparation necessary, the product mixing, thinning (type and amount), and application equipment recommended, and the temperature, humidity and minimum dry fall distance required to achieve the proper performance from the coating. The SDS will provide information concerning minimizing both worker and environmental exposure hazards when using the product, and for its safe preparation, application, and disposal. The importance of thoroughly reading, understanding, and following the recommended procedures explained in these critical documents cannot be overstated.
Dry fall coatings can be applied to new, clean, bare surfaces, and can even be used to prevent flash rusting of uncoated steel surfaces. Most manufacturers recommend initially either hand or power tool cleaning, and ensuring that the surface is clean, dry, and free of dirt, dust, oil, grease, rust, mildew, and release agents before application. Previously coated surfaces should be scraped, sanded, or wire brushed to remove all loose material and to feather any edges of old paint that remains attached, then thoroughly washed and dried before application. Safety precautions should be observed since preparation procedures have the potential to generate lead or asbestos dust from any old paint or insulation present. Any exposed bare surface should be primed before the final coat is applied; refer to the PDS for primer recommendations. Often, applying a small test patch of the new dry fall coating over the prepared surface is recommended to ensure good adhesion and proper performance.
For a detailed investigation of what can go wrong when inadequate surface preparation combines with high curing stresses withing the coating, read the KTA University article, “The Sky is Falling! Or is it Paint Chips? Failure of a Dry-Fall Coating Applied to the Interior of Previously Painted Metal Decking” by Rich Burgess, which can be accessed using the following web link:
Although most dry fall coatings can be applied using either conventional or airless spray equipment, some do specify airless only. Besides any specific equipment recommendations on the PDS (spray gun, tip size, hose diameter, pressures, etc.), it is also very important to follow any information given on the proper thinning, coverage rate, wet and dry film thickness, temperature and humidity ranges, and the minimum dry fall distance required during the application. Venturing outside any of these recommendations increases the chances of the product not performing as expected, possibly causing expensive rework and overspray claims. As an example, here at KTA we fielded several calls from a potential client who had much experience spraying a dry fall coating from one manufacturer with great success but could not obtain that particular product and had to use a coating from another manufacturer for a job. After many discussions concerning the problems he had with the application on this job, we discovered that he did not review the PDS of the new coating prior to application, and the recommended application parameters were quite different from the coating he had experience with. If there is one thing to take away from this article, it is to READ, COMPREHEND, AND FOLLOW THE PRODUCT AND SAFETY DOCUMENTATION THOROUGHLY BEFORE STARTING THE APPLICATION!
Laboratory Testing of Dry Fall Coating Performance
KTA was instrumental in the development of a standardized Practice for determining the dry fall performance of these types of coatings, which has been published by the American Society for Testing and Materials (ASTM) as test method D7868, “Determining the Dry Fall (Fog) Properties of Protective Coatings,” and can currently be performed by KTA on water-based dry fall coatings. Why is a standardized method necessary? Because once coating manufacturers have their dry fall coatings tested according to this method, their coating is given a qualitative attachment rating indicating how easy or difficult it is to remove the overspray. This rating can be used for a quick comparison of the relative ease of removal of the overspray of dry fall coatings from different manufacturers. The test also provides an independent verification to a coatings manufacturer and their clients of the suitability for use of a dry fall coating.
Briefly, the testing involves using airless spray equipment to spray a dry fall coating for 45 continuous seconds onto a target located 18 feet above test panels that are coated with a typical automotive basecoat/clearcoat system in 3-foot wide by 4-foot deep by 19-foot high enclosure. This is done under controlled temperature (60 to 80°F) and relative humidity (40 to 85%) within and around the enclosure. The spray equipment is set-up according to the coating manufacturer’s recommendations. The test is performed with the spray tip of the gun at two different distances from the spray target (2 ft. and 4 ft.) at the top of the enclosure, and the test panels that collect the overspray at the bottom of the enclosure are at two different temperatures (ambient and 122°F). The panels that have collected the overspray are then allowed to age for 24 hours at both ambient and 122°F temperatures. After 24 hours, the overspray is tested for ease of removal from the automotive basecoat/clearcoat surfaces either first by dry brushing (easiest to remove), or next by using a cellulose sponge and washing with a solution of car wash concentrate (moderate ease), or lastly by rubbing with a cotton cloth soaked with mineral spirits (most difficult). Each of these removal methods are rated on a scale of 1 to 4, with 1 indicating severe attachment (very little, if any, overspray removed) and 4 indicating no attachment (overspray completely removed). Depending on whether the color of the dry fall coating is light or dark, automotive basecoat/clearcoat test panels of the opposite color are used to easily view the overspray on the test panels.
If there are no attachment ratings according to the ASTM D7868 Practice listed on the PDS of the dry fall coating you are considering using, contact the coating manufacturer and encourage them to have this testing performed and their product rated. This will provide users with a quick guide as to how easy the dry fall coating will be to clean up after application, and how just much easier your job could be.
About the Author
Joseph E. Barush is a project manager/coatings application specialist with KTA-Tator, Inc. in Pittsburgh, PA