Going from crumpled metal to ready-to-paint requires
coordination between the metal and paint prep techs.
Panel repair often involves coating “roughed” metal in layers of plastic filler and primers.
The paint prep tech has two primary goals: to create an exact contour of the panel and proper adhesion of all materials. It all starts with the repair technician. Plastic filler is a part of most panel repair plans. Surfaces must be properly prepared before the application of plastic fillers. The recommendation of some vehicle manufacturers, such as Toyota, is to paint bare metal surfaces with epoxy primer before spreading plastic filler, but the makers of premium plastic fillers say that additives in their products make this unnecessary. Check product data sheets for specific recommendations. What is absolutely necessary, however, is that the surface be free of contaminants and provide a mechanical bite for the plastic filler. Eighty- to 180-grit sandpaper is generally recommended. Courser grits, such as 24- or 40-grit grinding discs cause unnecessary damage to the metal and do not improve the adhesion of the body filler. Grinding discs are intended to grind metal away, such as when dressing welds. They should not be used for removing paint. After the surface is properly sanded, primed with epoxy if required, and plastic filler has been spread and sanded to shape, the repaired area should be primed as soon as possible to ensure that the integrity of the corrosion protection is maintained. Steel and aluminum start to react chemically very quickly when exposed to the atmosphere. Even with just the normal humidity in the air, rust will start foring soon after the surface is exposed. Excluding moisture and air from contact with bare metal is the only way to prevent rust. The sooner primer is placed over the repaired area, the better.
Primers
Primer used to come in two colors—that was about the extent of a tech’s choices. Today, most paint shops have several primers to choose from. Which primer, or primers, is used on each job is an important decision. Primers serve several functions. One is to seal the surface against moisture and air. Another is to make things smooth. Yet another is to provide a surface for color coats to stick to. As surface coating science has gotten better, a single-purpose primer performs each of these functions best.
For maximum surface protection, the tech may choose a self-etching primer. This is a primer with a little acid in it to etch into the metal. It has tremendous adhesion qualities and protects the surface very well. But self-etching primers do not have fillers in them. They also are not usually stable in ultraviolet (UV) light, so they must be covered by a topcoat.
Epoxy primers are also very good at protecting surfaces and may be formulated for use as a sealer as well. Fillers are often somewhat porous, so a high-build primer good for block sanding may not be the best choice to put over bare metal. Any porous material allows moisture to get to the surface. Primers with fillers, or primer-surfacers, also do not always flow out to a nice slick surface. So already there are several primers on the bench: one for bare metal, one to fill the area so we can sand it smooth, and one to spray last so the surface will be nice and smooth for the color coats. Additional considerations include: the type of substrate such as steel, aluminum, plastic or composite; if it’s plastic, how flexible it is; or if the part must have a chip-resistant coating. Some primers are intended to be a reasonably good filler primer, yet are still rated for use over small areas of bare metal—such as those found around the edges of plastic filler on a repaired panel. Some primers can also be used as a chip-resistant coating.
The key to good performance of any paint system is training from the paint manufacturer, which makes it an easy task to select the best primers to use for each job. Check with your paint system manufacturer for listings of specific products for the specific repair task at hand.
Feathering
The interface between plastic filler and original paint is sometimes a problem. The technique of feathering ensures that this area is invisible in the finished repair. The process is not difficult, but must not be overlooked. Usually with a Dual Action (DA) sander, the paint is sanded with the tool flat on the surface until there is a wide, shallow taper from the original finish to the bare metal. When finished, a nicely feathered edge is very hard to feel when rubbing the panel. Many plastic fillers are now rated for use over stable original paint, but the paint must be feathered out with the recommended grit abrasive before fillers are applied. Again, follow the product manufacturer’s recommendations.
Blocking
After the primer-surfacer is cured, the surface must be blocked to make sure it is the proper shape. This is a much harder skill than it seems at first. The principle is simple: a flat block with sandpaper on it is used to sand the surface using a uniform pattern, typically a criss-cross “X” pattern. All the high spots are sanded down, but the sandpaper never cuts into the low spots. Eventually, the entire surface is level. In fact, choosing the proper shape and size block and using it correctly is something mastered only with much practice.
Masking
Primer is formulated to adhere to most surfaces, so anything not covered will become primed. Masking paper or plastic should be used to cover large areas. Tape is used to define the edges. The question of which parts get taped off and which need to be removed for refinish operations fuels an ongoing discussion among technicians, estimators and insurers. Paint companies insist that paint be sprayed so it lays down flat on the panel’s surface. Many warranties are void if paint coatings bridge onto adjacent parts. This means that many parts such as moldings, handles and anything with a gasket under it must be removed. The exceptions are parts that stand off enough from the painted surface to allow sanding, taping and spraying to get under the part, and those moldings that can be held off of the paint with a “rope” or plastic tape designed for that purpose. Otherwise, parts expanding and contracting with temperature changes cause the edge of paint to break off. This provides a path for moisture to get into the layers of the paint, causing failure of the paint film that will lead to corrosion.
Whose job is it?
One point of contention between some body and paint techs is how smooth the surface must be before filling primer is applied. This is a case of the state of the art changing throughout time. Thirty years ago, nitrocellulose lacquer primers were used to fill 24-grit scratches. However, 36-, 40-, 80-, and now 180-grit became the target during the past several years. Some of this has been the result of body fillers that are much easier to sand with finer grit sandpaper. Twenty years ago, using fine grit sandpaper on plastic filler just gummed up the sandpaper. Today, many plastic fillers can be sanded with 120- or 180-grit quite easily. Also, a large part of the change toward finer grits is due to increased concerns about corrosion protection and new paint chemistry that requires finer surface preparation. Heavy fillers in primer result in a somewhat porous primer. Primers with good sealing characteristics cannot fill deep sand scratches left from courser grit sandpaper. Ultimately, the primer data sheet instructions are what determine the final grit used to finish repaired surfaces before priming. With most priming systems, the recommendation is that surfaces be sanded to at least 180-grit before any primer is applied. Technicians must follow the recommendations of the priming system that will be used.
Which technician sands the repaired area to what grit and who blocks the final shape are decisions to be made by production management. Where the money is reflected on the estimate does not always coincide with best practices in the shop.
A thorough understanding
As with so many aspects of collision repair today, understanding the specific materials in use during the repair is the key to achieving outstanding results. Keeping your paint and materials product data sheet library up to date is essential. Establishing standard operating practices so each technician understands his or her responsibilities will keepproduction running efficiently.
