May 2000Finishes change--from both the vehicle maker and the paint maker--and this includes undercoats. For example, the Dodge Dakota is being treated with a new undercoat this year at a plant in Brazil. The coating is an impact- and chip-resistant material that is applied using a second E-coat dipping process. Also at the OEM level, new primer materials allow the direct application of topcoats to thermoplastic olefins (TPO) and other plastics that are difficult to finish.Because of changes such as these, refinishing technicians may no longer be able to use some undercoats and application steps they've become familiar and have had success with. New materials require some very specific paths and choices. To produce quality repairs, techs must keep abreast of today's undercoating materials, problems that can develop during application and how to choose undercoats for repair facilities.Through both emerging technology and government mandates, there have been two significant changes in undercoat technology. First, there was the emergence of materials that can be used for more than one process, such as primer-sealers and self-etching primers. Second, thermoset materials replaced thermoplastic coatings, such as lacquer-surfacer, that dry solely by evaporation of solvents. Thermosets have a higher solid content and usually don't allow penetration of solvents.Undercoating BasicsTo discuss changes to your current processes and selection of materials, it's necessary to review basic technology in the four undercoat families: primers, primer-surfacers, primer-sealers and specialty undercoats.The major function of a primer is to provide adhesion. These materials are all very rich in resins, usually have limited pigment volume and use what are called soft solvents. Various additives enable primers to provide additional benefits, such as rust inhibition, corrosion protection, high flexibility and weather protection.When applied in the proper thickness, primers rarely cause a lifting problem or surface reaction and dry relatively fast. Many product makers recommend that a primer be applied under body filler to enhance adhesion and to provide improved corrosion protection. It's also recommended to apply an epoxy primer under filler whenever making repairs on aluminum.These are some features of today's primers:Isocyanate-free--They are designed to reduce health risks and are made for technicians who have been sensitized to isocyanates and, therefore, are unable to use materials containing any isocyanates. Zinc-chromate free--Removing zinc and chromates eliminates anti-corrosion qualities but typically adds adhesion characteristics. Vinyl-based--Also called etching or wash primers, these may not contain chromates but may contain phosphoric acid. Self-etching--These primers contain phosphoric acid.Tintable--Adding tinting base colors allows the undercoat to closely match the topcoat color. Epoxy--These are two-part (2K) primers that are corrosion-resistant and also increase resistance to topcoat solvents or other chemicals. Ready-to-spray--These primers require no reduction or additives and are often reacted by oxygen from the air.Many primers have a combination of these qualities. Choose a type of primer based on the widest application of use.Primer-SurfacersPrimer-surfacers are used to fill or level minor imperfections. Although the name is always primer-surfacer, the major function of these materials is to fill. The name implies two materials, but primer-surfacer materials can't provide the same adhesion as a primer because primer had to be removed to make room for the filling agent. Thus, a primer-surfacer should only be used when filling is required. Although they provide a cushion for the topcoat, this material will add to overall film thickness. This can contribute to such paint problems as sandscratch swelling, sandscratches, solvent popping and cracking or checking.Similar to primers, this family of undercoats has a number of feature choices to consider, including the following:Single-stage--These are not catalyzed and are reduced with solvent. These materials are not as chemically resistant to the solvents that follow, and they are more likely to encounter sandscratch swelling, lifting and other problems. Two advantages of single stage primer-surfacers are that leftover material can be returned to the container and used later and that these materials don't use a hardener that contains isocyanates.Catalyzed--Most primer-surfacers used today are catalyzed. There's a wide selection of types, including acrylic, urethane and high-build or high-solids. You must study the features of each available material within the paint line used in your facility. Make a selection based on the compatible substrates and topcoats that are common in your facility.Waterborne--Waterborne primer-surfacers can be used on almost any surface, including properly treated bare metals, without causing a reaction. They also have the ability to withstand any material that is applied over it.SealersSealers or primer-sealers achieve two goals in one step: an innercoat bond and a topcoat holdout.The innercoat bond feature is the ability of a sealer to adhere to just about any material, including fillers, primers, bare metal, primer-surfacers and other materials used in the refinished repair area. Sealers also provide a surface that the basecoat can adhere to. The topcoat holdout feature prevents solvent penetration and reduces problems, such as solvent popping, sandscratch swelling, dulling, uneven dry or loss of adhesion. An additional advantage of applying sealer is providing a uniform color base for topcoat color development.In the family of sealers, there are again many choices. In addition to those listed under primers, the options include the following:Multi-function--These sealers can be used over bare substrate and can be highly flexible, or they can be used as a wet-on-wet clear bonding coat.Single-stage--Ready-to-spray, these do not require a catalyst, and unused material can be returned to the container. These are normally activated with the mixture of oxygen from the atmosphere.Tintable--These allow faster color development when matched with translucent topcoat colors. They also reduce the number of color coats required. This feature also minimizes the visibility of rock chips and other damage to the paint surface.Specialty UndercoatsSpecialty undercoats are usually supplied by specialty product makers and are often available in aerosol. Examples of these include the following:Plastic adhesion promoter--These are available in a variety of choices for use alone, added into another product and in many other applications. These should not be confused with the adhesion promoters used for plastic repair.Universal flex agents--These additives are used with some undercoats and topcoat colors to provide flexibility on plastics.Butyl coatings--Normally called rubberized undercoats, these materials provide soundproofing and weather-resistance to underside areas. Generally, these products are paintable.Metal treatments--Although not generally considered an undercoat, these treatment materials help build the strongest bond to bare metal.Other specialty materials include weld-through primers and anti-corrosion compounds.Preventing ProblemsProblems with applying undercoats today are mostly the result of improper mixing, application method or curing requirements. Consider the following:Mixing--Proper mixing, or reducing, allows flow and leveling. Reducing also allows the different parts of the material to stratify, or reach their proper placement in the coating. This problem can be remedied by following the product maker's recommendations.Application--Proper application thickness provides the material with enough internal strength to withstand penetration of solvents from additional coatings and provides adequate adhesion to the surface. (Note: When applying undercoats, understand that the heavier the material is, the smaller the fluid flow should be. Increase the atomization by decreasing the flow of fluid, thus achieving a smooth, level surface.)Curing--Proper cross-linking, evaporation and short cure are achieved in the recommended dry time. Dry times will vary greatly if heat is used. Allow a purge time to prevent skinning the material. Be aware of the time window. Coatings are allowed a specific time to dry before coating, and if allowed more than a certain time, they must be sanded or recoated.Timing IssuesAnother source of problems is where refinishing fits into the repair. The refinishing operation is always the last major repair, so painters are under the most pressure for quick delivery. But this time crunch can also lead to problems. The most common "time" problems are as follows: Solvent popping--These are small, pin-like holes that appear after the finish is applied. These are caused by solvent trapped in the undercoats when the material dries on the surface before the solvent can escape. This condition is caused by solvent or hardener that is too fast, coats being applied without proper flash, an improper spraygun or setting, not allowing a purge time before force-drying and too long of a force-drying time. Poor adhesion--This is usually the result of improper surface preparation, absence of the proper primer or incompatible materials. Blisters--These bubbles or dirt contaminants inside the material are the result of failing to take the proper cleaning steps before applying a material. Another cause of blisters is contamination in the compressed air system. Dull spots--Failure to use a sealer allows the topcoat solvent to be absorbed by the variety of materials it is being applied to, thus creating an uneven dry and gloss factor.Avoid these problems by allowing the times required for each material and step of the process. It also helps to use a slower solvent in the undercoat material. Contrary to belief, the material will dry faster when using a slower solvent. This process allows each part of the material to reach its position, do its job and allow the solvent to escape.Making ChoicesSelect undercoat materials by gaining the best knowledge of the materials available. By taking the information you've assembled about your needs, abilities, shop conditions, product lines and the types of repairs done, you can establish a list of materials you wish to use and a host of processes you want to follow. Develop a chart that includes the known factors and desired qualities, and then select the product that most meets those needs. Also, work with your supplier to gain a level of knowledge that will allow you to successfully complete repairs in repetitive fashion with little or no problems. Continue this relationship so that you are informed about changes in the use of a material.Changes in finish materials will continue and require constant reviews of the technology used in collision repair. For example, you will soon be refinishing a higher number of vehicles built with aluminum, additional plastics and even materials that are unheard of right now. These changes will undoubtedly require repair-process modifications--beginning with the undercoats.This article originally appeared in the I-CAR Advantage, a technical newsletter published six times per year for the collision industry. A one-year subscription is $29 in the United States and $34 in Canada. Please call (800) I-CAR USA or (800) 565-ICAR in Canada for details.Marc Essig is an education consultant for the industry who devoted 29 years to collision repair instruction at Clackamas Community College in Oregon City, Ore., serving as chair of the automotive department for 23 years. An I-CAR instructor, he is also finishing his six-year term as a member of the I-CAR Education Foundation's Board of Trustees. Essig also serves as an instructor for PPG Industries and Valspar/Sunbelt Paint Co. and is a certified weld examiner for the American Welding Society's MIG weld test. He is ASE-certified in collision repair-non-structural, collision repair-structural, and automotive refinishing.
About the Author
Marcus Essig
Marcus Essig retired from Clackamas Community College in Oregon City, Oregon after 29 years teaching and directing the Automotive programs. Marcus received his formal education at Oregon Technical Institute in Automotive Technology, Collision Repair Technology, and Welding/Metals Technology, continuing at Oregon State University in Trade and Industrial Education. Currently he is the Lead Instructor for I-CAR in Oregon and also is the Administra-tor for both the Steel and Aluminum Weld Qualification Test.
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