The concern today with removing coatings is restoring the vehicle's corrosion protection after the repair is complete. The key to this is paying attention to the differences between paint removal methods.
Paint removal is an old task that has recently taken on some new and different twists. The concern today resulting from the removal of the color coats and undercoats stems from the importance of replacing the corrosion protection. With the vehicle manufacturers providing extended anti-rust warranties, it is vital that technicians understand the differences between paint removal techniques.Starting PointDeveloping answers to the following questions will assist you in conducting a repair in reasonable time while maintaining quality:1. Why do I need to remove the coating? Removal may be as simple as sanding off a checked or oxidized clear coat and repairing it with the application of new clear. Stripping, or removal, is done to allow repairs to the sheet metal or structural components without the paint becoming a contaminant in the repair. The most common reason for removing coatings is to apply plastic filler.Structural repairs often require cleaning off the coatings in a weld zone, where paint materials easily contaminate a weld and result in severe porosity, which can weaken the repair. Panel replacement where bonding is the form of attachment may require that some of the factory material not be removed. This will vary, based on the adhesive manufacturer and vehicle repair guidelines, directing non-removal of the E-coat primer and/or galvanized coating. Generally, the adhesive with corrosion protection qualities determines the removal requirements. Be sure to read and follow the material instructions.2. What kind of material am I removing? The modern vehicle is produced using high-quality urethane coatings that are not easily removed compared to the old style of lacquer or acrylic enamel. Yet, these new coatings are extremely thin and leave little room to grind or sand away selectively. Beginning at the top, the clear coat is the thickest of all coatings and is removed with a fine sandpaper abrasive. Under the clear lies the very thin base coat color, which can be easily taken off.Next, factory primer-surfacer, which is totally different than the field-applied product, will give resistance to sanding and requires a coarser abrasive in the 120 to 150 grit range. E-coat primer is the last of the coatings, and it is not an easy product to sand.On many vehicles, you will find a galvanized coating under the E-coat. This is most commonly applied to skirts, aprons and structural parts that are subject to retained moisture. It is preferable to leave this coating on during repairs, including welding, where the zinc will not affect the weld quality. But this may require a little extra heat from the welder.Removal StylesUsing chemical paint remover can provide many benefits. We generally think of chemical remover, or stripper, as a product used for large areas or a complete strip leading to restoration. But some common repairs are completed quicker and better using a stripper, such as the removal of coatings in a weld zone where you do not want to take off the galvanized coat or want to grind and remove metal, which can weaken the joint. Remember that all grinding and sanding operations leave bare metal exposed that combines with atmospheric oxygen and water, resulting in ferrous oxide-rust. This rusting action begins when the metal is first exposed, but you can avoid this destructive condition by using a stripper.Chemical products are usually packaged in bulk and aerosol. The strength of the product varies and should be tested before a repair is conducted. Don't rush this process because the chemical needs to sit long enough to penetrate and break the bond of the coating to the substrate. Severe wrinkling will occur when the bond has let go. Most products are more easily removed if they are left on until almost dry. Multiple applications may be required if you are removing paint that was applied over a factory finish. If you watch carefully, you can even leave the E-coat layer, reducing the re-treatment process that follows.Dip Strip is a process where the component, panel or entire vehicle is immersed in an acid bath. This is a common method of stripping a vehicle for complete restoration. When using the dip process, it is vital that the corrosion protection be replaced as quickly as possible. Liquid metal conditioners will provide the best in cleaning, etching and conversion coating protection. The product is also not selective and will damage moldings, upholstery, wiring and other parts of the vehicle.When using chemical strippers, it is very important to take proper safety precautions. Chemical strippers contain materials that are hazardous to the human body. These products are caustic and cause severe skin burns, respiratory problems, and partial or complete blindness. Follow the manufacturer's safety recommendations completely and take no chances.Mechanical removal-laying into a surface with the old grinder-does seem to be the fastest way of taking off a material, but hold on. With today's vehicles having very thin paint applied to thin metal panels, the days of grabbing the Milwaukee 9-in. grinder with a 16-grit disc are history. My old workhorse is lying under the bench and is only brought out to do heavy weld-cutting or equipment repair. The tool selection today includes 5-in., high-speed grinders/sanders, dual-action sanders, die grinders and jitterbug sanders. In addition, the selection of abrasive types continues to grow and become more specialized.Yes, we still need to take the grinder to a panel and remove large areas of paint in preparation for plastic filler application, and the 5-in. high speed is the way to go. Just remember that the abrasive is going to remove metal as well.Tests have proved that filler sticks better to fine scratch marks than coarse cuts and sticks even better to a surface that is primed prior to filler. Use the abrasive that will remove the paint easily and leave small cut marks. Never use a closed-coat abrasive disc because it will clog immediately.Selective removal has become quite common with the advent of E-coat primer, galvanized panels and adhesive bonding. Your jobber carries a wide variety of specialty wheels, discs and pads that will adapt to most of the power tools in your toolbox, providing you with a broad range of repair possibilities. Most of the pads and wheels are made of woven plastic material, but there are some with materials like die grinder wheels and cut-off discs.Also included in the selective range is the compound family. Liquid compounds can be used for removing oxidation and other defects, including industrial fallout, organic contamination and minor crazing.Shipping primer that covers many aftermarket replacement panels must be removed before refinishing. This primer can often be removed with a lacquer-type solvent or dual-action sander using 100- to 120-grit abrasive. A simple solvent test is done to determine if the primer needs to be removed: Rub an inconspicuous spot with a rag saturated with lacquer thinner. If the primer comes off onto the rag, the primer must be removed.Aluminum components must be treated differently. Never use the same abrasive on aluminum that you previously used on steel. Cross-contamination will occur by depositing any steel particle into the aluminum surface. Galvanic corrosion will take place and may not surface until long after the repair is completed and the vehicle is delivered to the customer.Create a separate small toolbox that contains "Aluminum Only" repair items, including abrasives, scrapers, discs, drill bits, cut-off wheels and any item that abrades the aluminum surface.The last and least desired tool is the razor blade or scraper, even though they have their place in the scheme of paint removal.Safety precautions with mechanical removal techniques are also a concern. Mechanical removal generates hazardous dust, flying particles and sharp edges that can harm the technician and others. Always use eye protection, wear an appropriate respirator, protect adjacent panels and glass, and warn those close to you of your intent. Many paint coatings also still contain lead and chromates, both of which are known carcinogens.Blasting or impact removal, has taken on many new methods during the last few years. Traditionally, blasting was the main step in complete restoration or paint and rust removal on older vehicles. There were many problems with blasting: The process removes metal, damages glass, dulls chrome moldings, crawls into every nook and cranny, requires immediate application of a primer and seems to always come out of hiding when you apply the last coat of color or clear. In addition, sand blasting makes a horrible mess that is difficult to clean up.Small portable units began showing up, but they, too, had all the disadvantages, except the mess was smaller. Then in the 1980s, a new unit that was self-contained brought about a revival of blasting. These units not only captured the sand and waste but also used a new media, such as plastic, soda or nutshell that reduced many of the major problems with sand blasting. The largest advantage is the minimal amount of base material that is removed. By regulating the air pressure, the technician can even control the depth of removal, leaving the galvanized coating.Tremendous advantages are gained by the new self-contained system when used to prepare an area that is flat and that would warp with any grinder, thick coatings where a previous repair had been done, and areas next to glass or expensive gold-plated moldings.Following any form of blasting, corrosion protection needs to be replaced. Because blasting leaves small pits in the surface, never use metal conditioners to treat the metal. Acid cannot be removed from the small pits, and the metal will soon begin rusting. Apply either an epoxy or a self-etch primer.Like mechanical removal, this process also generates a hazardous environment. Be sure to protect your eyes, lungs and skin.The Last StepAfter removing the paint and protective coating and after the repair process is completed, you must replace the corrosion protection of the vehicle. Here, too, there are many products to choose from, based on the exposure, type of repair and process to follow. The following are a few of the more common materials:Weld-Through Primer-This is applied to the mating flanges of MIG- or resistance spot-welded joints. The newer forms even contain a quality self-etch or epoxy feature that does not limit its application to only the bare metal.Metal Conditioners-These provide the better of the preparation processes, yet they are rarely used because it's a wet process. Never use these products in an enclosed area, behind moldings or on flange joints.Epoxy Primer-This is the most common material that provides quality protection to the base metal, a good bonding surface for the next coating and may have many other applications.Self-Etch Primer-This is now becoming very popular for its protective quality. It has some limitations but is worth having and using for quality replacement of the factory material. (Warning: This material contains acids, phosphates or chromates in order to be self-etching. Apply it in properly vented areas, preferably a paint booth, and wear respiratory protection.)Zinc-Rich, or Galvanizing Primer-This has the properties to replace factory galvanizing and should be used when the original panel was so coated. (Warning: This product must also be applied in the paint booth. Be sure to use proper respiratory protection.)Vinyl Wash Primer-This is best applied when completing repairs to aluminum. The vinyl resin provides superior adhesion to the surface.Paint removal requirements are going to be around for many years and, undoubtedly, many changes will take place in the methods and the materials used. It is to the advantage of the technician to remain aware of these changes by reading trade magazines and product flyers and by attending jobber clinics, workshops or I-CAR classes. And remember: Only remove the minimum amount of coating to properly conduct your repair.
