The automotive body provides protection for the engine, power train components, operator, and any cargo or passengers. At the same time, it adds strength to the frame and provides adequate vision for the operator. Last but not least, the body design provides a pleasant outward appearance.
Part of your job as a automotive technician may be to perform body maintenance of the vehicles assigned to your command. In order to perform this task, you must know the procedures used for straightening fenders and body panels. Preparation and painting of the vehicle are other important tasks associated with this responsibility.
Regardless of whether the vehicle is in need of extensive bodywork or has a dented fender, it is desirable to have a number of special tools. One of the most important tools required to repair heavily damaged areas is a portable hydraulic jack (porta-power) (Figure 12-54). The porta-power is provided with a number of adapters or accessories that will allow you to use it in many types of body repair work. When applied as shown in Figure 12-55, this tool will force the damaged area to return to near original shape and save many hours of labor.
Figure 12-54 — Portable hydraulic jack.
Figure 12-55 — Pushing a dent out using a portable hydraulic jack.
Spoons, dinging hammers, and dolly blocks are the common working tools found in the body shop (Figure 12-56). These tools are used to remove dents and smooth out and shape damaged areas.
Make sure the surfaces of the spoons, hammers, and dollies are free from scratches and/or dents. Surface defects on these tools will cause similar defects in the sheet metal they are used on. To remove surface defect on these tools, use a file and fine grit sandpaper until you have a smooth surface.
Figure 12-56 — Hammers, spoons, and dolly blocks.
With these tools and experience you will be able to remove the dents and creases while restoring the body to a like-new condition. The ease and speed with which you can straighten the sheet metal is dependent on starting the repair work at the right point and correctly using the tools. If this is done, the amount of “dinging” (light tapping of the metal with a hammer) required to remove the dent is reduced considerably. As metal is dinged and formed, a certain amount of stretching occurs. This causes additional work when nearing completion of the repair. Always remember, when straightening a damaged panel, remove the damage in reverse order of how it occurred.
Before attempting any body repairs, scrape off any undercoating or foreign matter located in the area to be repaired. Dirt or undercoating will cake on the dolly block. No amount of hammering will produce a smooth surface when this occurs. Next, to protect the hammer, make sure the outer side is clean. Without prior body repairing experience, a mechanic will usually start applying pressure at the spot where the panel was struck first and is depressed the most. The correct method is to apply pressure at the ridge farthest from the point of impact. To understand the procedure clearly, refer to the damaged panel in Figure 12-57.
Figure 12-57 — Proper method for repairing a damaged body panel.
Assume that the original form of the panel is shown as the dotted line. Point Y is where it was struck, and X is a ridge that was formed last. With the use of a spoon and hammer or mallet, place the spoon on the ridge (X) and strike it with the hammer. Aim your hammer blows directly at the ridge (X). By following the ridge with the spoon and hammer, you will find that the ridge will gradually disappear while the major portion of the depression at point Y will spring back and very closely resemble the original contour of the panel.
Using a dolly block with the same general curvature as the panel, place it under the panel at point O and strike the dent as shown. In this way, the dolly block acts as a hammer and raises the dented portion to the original contour as the dolly block is gradually moved toward point Z. The most common mistake made by an inexperienced body repairman is trying to do all the work with one blow of the dolly. All that is necessary of the hammer or dolly is to press the metal back into position. A number of light blows with the hammer or dolly are better than a few heavy ones. Heavy blows result in the metal stretching excessively during the straightening process. This requires that the panel be shrunk later to remove bulges.
When working with the hammer, apply blows rapidly with a pulling action so the hammer tends to slide as it contacts the metal. Above all, do not try to rush the job by striking the metal too heavily. Figures 14-58 and 14-59 show the procedures for removing dents when performing bodywork. Use of a flat-faced hammer should be confined to the flat or nearly flat surfaces and the outside of curved surfaces. Hammers with crowned faces are for use on concave surfaces only.
Figure 12-58 — Using a hammer and dolly to remove a dent.
Figure 12-59 — Using a spoon and hammer to remove a dent.
Generally, a severely damaged panel will be replaced or repaired by cutting out the damaged area and replacing it with sheet metal. Should you have to repair a heavily damaged body panel, there are a few things you should consider before starting the job.
The first and most important consideration is to determine the direction of force that caused the damage. This will enable you to use the hydraulic jack and its attachments to push the panels back into a near original position. At the same time, the braces holding the sheet metal will move back to their original position and allow access to any bolts and fasteners that must be removed to disassemble the damaged body parts.
Once you have reached this point, you must determine if the damaged panel is to be repaired or replaced.
If you decide to replace the damaged panel, make sure any braces that support the panel are ordered also. New braces will assist in aligning the new panels with the rest of the body. Should only a portion of the damaged panel be replaced, an oxygas cutting and welding outfit or a cut off wheel will be required to remove the damaged portion and weld the new sheet metal into position.
Complete instructions on the use and care of oxygas cutting and welding outfit are contained in the current edition of the Steelworker training manuals. Consult these manuals for the proper method for adjusting and using the cutting and welding tips.
The procedures for replacing a portion of a damaged panel are as follows:
Before actual painting begins, it is essential that you prepare the surface for the paint by removing all traces of wax, grease, oil, and dirt. If the paint on the vehicle is of poor quality or deteriorated, remove it. In this final preparation of the body before applying paint, you have several methods to choose from. The method that is selected depends on the condition of the existing paint, the equipment available, and the quality of the desired finished product.
If the paint on the vehicle is in good condition (with good adherence and without surface defects), go over the surface with fine sand paper (usually a 400 grit sand paper is recommended).
To remove the paint to the metal, start with a coarse grit (usually 36 grit) and work your way up to a fine grit to remove any scratches in the metal.
If the paint is to be removed from only a portion of the panel, taper the sanded area down into the old paint to produce a featheredge. Follow up with a 150 grit paper in a block sander, and complete the featheredge by water sanding using wet or dry paper of 280 or 320 grit.
Some manufacturers of abrasive paper advise different grits with variations of the above procedure. Follow the instructions of the manufacturer.
For removing paint from the entire vehicle, sandblasting is the preferred method. Among the advantages claimed for the sandblasting method are speed, low cost, and a surface that has good paint adherence.
After removing the old paint, clean the surface with a cleaning agent. If none is available, a lint-free cloth saturated with paint thinner can be used to wipe down the surface. This will help the new paint to adhere to the metal and will remove dust and other foreign matter.
Apply the primer coat as soon as possible after the paint is removed. This is particularly important when the surface has been sandblasted because the surface is practically in a raw state and quickly starts rusting.
Equipment shall be repainted when inadequate protection is afforded against rust and corrosion. Equipment will NOT be repainted merely to change the color or gloss characteristics if the finish is serviceable. Spot painting, in lieu of completely refinishing previously painted sections, should be done whenever practicable. Bare surfaces of body sections and sheet metal exposed by deterioration of paint or by accidents should be spot painted immediately to prevent deterioration of the metal.
When using any paint product, particularly lead-base paint, all current health and safety regulations should be strictly enforced. Contact the activity health/safety department/office to obtain all applicable regulations and instructions pertaining to a safe painting environment.
Before painting, apply a coat of primer to prevent peeling and flaking where bare metal is exposed. The primer serves as a bond between the paint and the metal of the vehicle. Each coat of primer that is applied should be allowed to dry and must be sanded lightly between coats. There may be occasions to use two coats of primer, but normally one coat is adequate.
Shake or stir paint and primer thoroughly, thin it with a thinning agent, and run it through a strainer or filter when using a spray gun. One of the "musts" of spray painting is that the paint should have the correct viscosity. This can be determined by following the instruction on the paint can. Too many painters determine the viscosity by the rate at which the paint runs from the stirring stick. This can lead to plenty of trouble, since only a slight change in viscosity can spoil an otherwise good job. This happens because the amount of thinner not only determines the thickness of the coat but also influences the evaporation rate between the time the material leaves the spray gun and the time it contacts the body panel.
High viscosity paint produces paint sag and orange peel, while low viscosity paint produces improper flow out and waste of thinner. To avoid these problems, take care to measure the proportions of thinner and paint accurately in a graduated measuring cup.
The temperature at which the spraying is done is also an important factor in turning out a good job. This applies not only to the temperature of the shop but also to the temperature of the vehicle. Shop temperatures should be maintained at approximately 70°F. Whenever possible, bring the vehicle into the shop well in advance of painting so that it becomes the same temperature as the shop. Spraying paint on a surface that is too cold or too hot from being in the sun will upset the flowing time of the material and will cause orange peel and poor adherence to the surface.
Another important factor in doing a good job is the thickness of the paint film on the surface. Obviously, a thick film takes longer to dry than a thin one. As a result, the paint will sag, ripple, or orange peel. Ideally, you should produce a coat that will remain wet long enough for proper flow out, but no longer. The amount of material you spray on a surface with one stroke of a gun will depend on the width of the fan, the distance of the gun from the sprayed surface, the air pressure, and the amount of thinner used.
In addition, the speed of the spray stroke will also affect the thickness of the coat. The best procedure is to adjust the gun to obtain a wet film which will remain wet only long enough for good flow out. Get the final thickness by spraying an additional coat after the first one has dried.
Nearly all standard spray guns are designed to provide optimal coverage when held at a distance of 8 to 12 inches from the surface to be painted. When the gun is held too close, the air pressure tends to ripple the wet film, especially if the film is too thick. If the distance is too great, a large percentage of the thinner will be evaporated in the spraying operation. Orange peel or a dry film will result because the spray droplets will not have opportunity to flow together.
It is imperative to hold the spray gun at the specified distance from the work. In addition, do NOT tilt or hold the spray gun at an angle. Also, never swing the spray gun in an arc, but move it parallel to the work. The only time it is permissible to fan the gun is when you want the paint to thin out over the edges of a small spot.
Another ingredient that is sometimes added to the paint is "hardener." This substance causes the paint to set and dry much more rapidly than normal. Because a small amount of hardener is all that is required, the instruction on its use must be followed closely. Mixing paint and adding hardener are two critical parts of painting vehicles. Use of the wrong type of thinner, paint, or excessive hardener will cause the paint to fade, peel, or blister within a short period of time after completing the job.
Epoxy fillers (body fillers) are simple to use in that the body portions do not have to be straightened as closely as when making repairs without it. By using the manufacturer’s instructions, you can apply body filler over rough places and form it with a body file or sanding until it conforms to the desired contour. The advantage of using body filler lies in the fact that a badly damaged vehicle can be returned to a like-new appearance quickly and with a limited amount of metal straightening. Additional, the use of thinner metals in the bodies of modern vehicles makes it difficult to reform panels into their original shape. Should you have an opportunity to use an epoxy filler, the recommended thickness of the filler should be kept to approximately 1/8 inch. If more is required, it should be applied in coats and allowed to dry before applying the next coat. Do not exceed an overall thickness of 1/4 inch.
In this chapter, you were introduced to the automotive chassis and its components. You learned about the springs, shock absorbers, and other components that allow the vehicle to pass over uneven terrain. You also learned about the steering mechanism and how it controls the direction of vehicle travel. We further discussed how to complete some body repair and the techniques used to restore body panels. You learned about how these systems operate and how to make adjustments and some repairs. This information will enable you to be a better Construction Mechanic when you have mastered the knowledge of these systems.