Research Bulletin 1990-1

  

SMC Repairs for the GM 200 Series Van

Introduction

General Motors has recently released a completely new mini-van. Three models are offered: the Chevrolet Lumina, Oldsmobile Silhouette, and the Pontiac Trans Sport, Photographs 1, 2, and 3.

This vehicle is a unibody design, but is unique in that most of its exterior body panels are formed from SMC, or Sheet Molded Compound. Another unique feature of this particular vehicle is that all exterior panels, except the front fenders, are attached with adhesives as opposed to mechanical fasteners. This type of fastening system provides body stiffness and added corrosion protection. Since this is the first high volume vehicle to incorporate SMC panels which are adhesively bonded to the steel subframe, Tech-Cor felt it was necessary to research the repair of such construction methods and provide this information to the insurance and body repair industries.

Tech-Cor would like to thank General Motors for providing a pre-production body shell, repair parts, and assistance which helped in producing the following research.

In an effort to increase the utilization of this document, Tech-Cor felt it was necessary to include detailed background and repair procedure information. We hope that this information will save you time and effort, and provide answers to questions which may arise.

Questions of Importance

What Is SMC?

Sheet Molded Compound (SMC) is a glass fiber reinforced thermoset plastic. The name SMC, as with RIM (Reaction Injection Molding), not only identifies the basic material composition but also describes the production process. Panels are produced by taking a charge or sheet of SMC and compressing it in a heated mold. The pressure and heat cause the material to fill the mold cavity. The mold temperature starts a chemical reaction which transforms the putty-like charge into a hard rigid panel. Panels used on the GM 200 are composed of:

The charge consists of random, chopped glass fibers encased between two equal thin layers of paste. The paste is produced by mixing the resin, calcium carbonate, and other materials.

Can Conventional Fiberglass Reinforced Plastic (FRP) Repair Materials Be Used?

SMC, though appearing similar, is not conventional FRP. The repair techniques may be similar, but composition of the SMC and the materials used for repairs are quite different. Many adhesives and cosmetic repair materials have been tested by General Motors. At this time, only two adhesives have been approved for repairs associated with the SMC panels. Ashland Pliogrip and Lord Fusor Body Panel Repair Adhesive are approved adhesives for bonding the panels to the steel subframe. Lord Fusor Body Panel Repair Adhesive is the only GM approved material for cosmetic repairs. Ordinary polyester fillers should not be used as in time adhesion is lost and the repair will fail.

Where Are The SMC Panels Located On This Vehicle?

All exterior panels are made of SMC and are bonded to the steel subframe. There is one exception to this, the front fenders are made of reinforced polyurea and are attached to the subframe by bolts. The front and rear fascias on all models, and the side skirts of the Pontiac Trans Sport are made of urethane, Photographs 4 and 5.

Are Special Tools Required To Repair SMC?

When using GM approved adhesives you will find they are packaged in a cartridge which requires a Supermix Pneumatic Gun for dispensing. This type of packaging is used to assure a consistent uniform material mix. When purchasing the gun specify the material(s) which will be used as different plungers are needed depending on the required mix ratio. When using any pneumatic dispensing equipment start at a low pressure and gradually increase to recommended work pressure.

What Is The Basic Procedure For Restoring Panel Surface?

At this point in time there is one proven process which will restore the SMC panels to a Class A finish. The process consists of layering Lord Fusor Body Panel Repair Adhesive and unidirectional fiberglass mat followed by heat curing. Unidirectional mat is used as it provides strength and helps obtain desired surface finish. Unidirectional mat is chosen over conventional fiberglass mat because it has open space between the fibers which allows the repair adhesive to envelop each fiber. Encasing the mat in the repair material provides strength. The mat also affects surface finish by reducing the amount of material involved in the repair which decreases the overall amount of shrinkage. Most materials after 24 hours at room temperature are not completely cured and will experience a small amount of shrinkage until full cure. If this small amount of shrinkage occurs after the repair is finished, surface finish blemishes may arise. Heat curing at 160 - 200 degrees Fahrenheit after each step will bring the adhesive to full cure in 30 - 40 minutes and eliminate any shrinkage that could cause surface finish problems.

Is Corrosion Protection Restoration A Concern With This Vehicle?

With the panels of this vehicle being made of SMC, it might be thought that corrosion protection restoration of the steel structure is not of great concern. This is not true, corrosion protection restoration is necessary as the adhesive bond is the only seal the interior has against exhaust, moisture, and gas fumes. If the structure rusts around the adhesive bond areas the environmental seal and body strength gained from the bonded panels can be lost. Application of a two part epoxy, zinc rich, or equivalent primer to bare or disturbed metal in accordance with Tech-Cor Research Bulletin 1986-7 Restoring Corrosion Protection During Vehicle Repair will provide adequate protection and adhesion.

Safety, What Protection Should Be Considered?

To ensure the health and safety of the technician and surrounding employees, dust control should be addressed. At the minimum the technician needs some type of respiratory protection such as a supplied air system or a particle mask. Use of cutting and sanding tools that control dust will also help protect the surrounding employees. An isolated area dedicated to repairs of this type might be the best solution for the dust control problem. A down draft prep station is another viable alternative.

Damage Diagnosis, Is It Just Like Steel?

SMC, because it is a composite, reacts very differently to impact than sheet metal. Because of this, SMC panels can mask the severity of an accident. The adhesive bond lines, interior structure of the doors and liftgate, and steel structure all need to be carefully inspected to get a true damage assessment. Close inspection may require partial removal of the interior trim.

Because SMC can fail in various ways, the extent and type of damage will determine the repair method.

The following identifies types of damage and the methods required to perform their repairs:

  1. Gouge - Penetration into panel but not through. Application of body panel repair adhesive required.

  2. Puncture - Damage has penetrated completely through the panel. Damage confined to one general area, a panel section not required. A backer panel must be bonded in from behind.

  3. Panel Splice - Panel damaged in several areas, not practical or cost effective to repair. Section in part of side panel. Backer panels required at all seams. A rear quarter panel splice will probably be performed most frequently.

  4. Full Side Panel Replacement - Damage severe enough to warrant full side panel replacement. Backer panels required at pillar seams.

  5. Re-skin of Door or Liftgate - Outer skin severely damaged but inner structure unchanged. Remove outer skin using heat gun and bond on replacement skin.

SMC Repair Methods

Gouge

  1. Remove paint from the repair area.

  2. Taper the edges of the gouge to accept the repair adhesive.

  3. Apply the body panel repair adhesive.

  4. Heat cure repair area. Cure time may vary with the amount of material used. An infrared heat source was found to work well.

  5. Allow area to cool and sand.

  6. Apply second coat of adhesive if required and heat cure.

  7. A sprayable polyester primer may be applied to block out the repair area.

  8. Refinish with appropriate paint system.

Puncture

When a puncture occurs in an SMC panel, the glass fibers will be damaged causing the area to lose strength. To return this strength a panel must be bonded to the backside of the panel. Before starting to perform a puncture repair two things need to be determined. First, is scrap material of similar contour available large enough to make a backer panel 2 inches larger than the puncture on all sides. If none is available then a backer will need to be fabricated, see Fabrication Of Backer Panel. Second, is there enough access behind the panel to perform the repair or does the interior trim need to be removed. The following two repair alternatives should provide enough information to allow the majority of all puncture type repairs to be performed:

Scrap Material Available For Backer Panel And Rear Of Panel Accessible Without Removal Of Interior Trim

  1. Cut away damaged area. A rectangular shaped opening will aid in the placement of the backer panel. Leave a small radius in each corner as this will reduce stress and the possibility of future stress cracks. A mini air saw or tin snips were found useful.

  2. Remove paint from repair area using 80 grit sand paper.

  3. The outside edges of the panel opening need to be tapered approximately 20 degrees with the panel surface. The taper should continue through the panel bringing the edge to a point. This is done to increase the bonding area for the cosmetic repair material.

  4. Using a piece of scrap SMC of similar contour cut a backer panel which is 2 inches larger on all sides of the repair opening.

  5. Check fit of backer panel. This can be done by feeding the panel diagonally through the panel opening.

  6. Prepare all bonding surfaces by cleaning with soap and water, and letting dry. Sand the backside of the panel and the backer panel to expose fibers. Wipe away the residue with a clean dry cloth.

  7. Drill two small holes in the center of the backer panel and fish a strand of wire through the face, out through the back and back through to the face. This wire will be used to pull the backer flush when bonding.

  8. Apply a 1/4 to 1/2 inch bead of adhesive around the edge of the backer.

  9. Feed the backer through the panel opening diagonally and then center.

  10. Pull on wires until an even bond thickness is achieved. Using a piece of wood or rod, tie the backer in place. Wipe away any excess material which may have squeezed out between the bevel and the backer.

  11. Heat cure the repair area, approximately 30 to 40 minutes with an infrared heat source placed 36 inches from the panel surface. Do not heat the panel to a point where it becomes too hot to touch. Heat curing will not only increase work productivity but will also remove shrinkage which can cause surface finish problems.

  12. Once the backer has cooled, scuff sand complete repair area and wipe or vacuum clean.

  13. Apply a 1/8 inch layer of adhesive to the face of the backer panel. On top of this place a piece of unidirectional mat which is the size of the exposed backer panel face.

  14. Lay a piece of wax paper over the mat and roll with a saturation roller until the mat is thoroughly encased by the adhesive. Peel off the wax paper. The wax paper is used to keep the roller clean.

  15. Apply a second layer of adhesive over the mat. Over this apply a piece of unidirectional mat which is slightly larger than the first.

  16. Apply wax paper and roll until mat is encased. Remove paper.

  17. Apply a third layer of adhesive over the complete area. Apply wax paper. Starting from center and working outward, spread adhesive with a plastic spreader to form contour.

  18. Heat cure repair area. Wax paper can easily be removed after surface hardens.

  19. After surface cools, sanding can start. Be careful not to undercut the SMC as the repair adhesive and SMC sand at slightly different rates.

  20. If final thin coats of adhesive are required, make sure to heat cure before proceeding. These thin coats may only take 20 - 25 minutes to cure with heat.

  21. Allow area to cool and sand to prepare area for finish.

  22. A polyester primer may be used to block out the repair.

  23. Refinish with appropriate paint system.

Fabrication Of Backer Panel

  1. Remove necessary interior trim and cut away damaged area.

  2. Determine the size of backer panel required. Remember it should extend 2 inches beyond each side of the repair opening.

  3. Remove paint from repair area using 80 grit sand paper.

  4. Taper the outside edges of the repair opening at approximately 20 degrees with the panel surface. Make sure the taper comes completely through the panel leaving a point at the panel edge. This is done to increase the bonding area for the cosmetic repair material.

  5. Sand the bonding area of the backside of the panel until fibers are exposed. Wipe away remaining residue with a clean dry cloth.

  6. Cut three pieces of unidirectional mat the size of the required backer.

  7. Provide a surface to build the backer. In our research, aluminum repair tape was used to construct a sheet one inch larger than the required backer size. Wax paper is also an alternative.

  8. Apply a layer of panel repair adhesive on the build surface the required size of the backer panel.

  9. Place a piece of unidirectional mat on the adhesive.

  10. Place a sheet of wax paper over the repair area and roll with a grooved saturation roller. This will force the adhesive through the mat. By encasing the mat in adhesive you are in essence fabricating SMC.

  11. Remove the wax paper and repeat steps #8 , #9 and #10 two more times.

  12. Apply a final coat of adhesive and spread into a uniform layer.

  13. From the inside of the vehicle apply the backer, Photograph 6, to the backside of the panel. If aluminum tape was used to build the backer the exposed tape will hold the backer in place until it has cured. If wax paper was used, masking or duct tape will hold the backer in place until it cures.

  14. Refer to previous repair Scrap Material Available For Backer Panel and Rear Of Panel Accessible Without Removal of Interior Trim. Start at step #11 and proceed to conclusion.

Rear Quarter Panel Splice

  1. Remove interior trim, rear side window, rear fascia, and tail lamp.

  2. Decide where to section the panel. General Motors will service rear quarter panels from the front edge of the wheelhouse rearward. A typical quarter section would start at the midpoint of the wheelhouse opening and extend rearward. Be sure to section fore or aft of the mill and drill pads. Photograph 7 shows the location of the mill and drill pads for the rear quarter of the driver side, the rear quarter passenger side locations are similar.

  3. The first cut is made vertically from the wheelhouse to the window opening. The cut is straight except for the offset at the top of the panel. The offset is being cut because of an SMC reinforcement. This reinforcement will act as a backer to support the splice. A pneumatic urethane cutter with a radial blade attachment, a mini air saw, and air shears all were found useful for cutting SMC without throwing great amounts of dust. Care must be taken when cutting not to disturb the steel structure.

  4. A second cut is made horizontally at the rear corner pillar.

  5. A decision must now be made because the rear of the quarter is covered by the liftgate surround panel. Three options to free the rear edge of the quarter panel are described as follows:

  6. Cut the section of the quarter panel, and remove. This is done to provide access to the adhesive bondline.

  7. Remove the mill and drill pad fasteners. Two nuts are located at the top edge of the panel and must be removed from the interior. Two screws are located at the bottom edge of the panel.

  8. The adhesive bond must be broken to remove the remaining panel from the steel structure. Using a heat gun at approximately 400 degrees Fahrenheit, heat the outboard section of the panel to be removed. Concentrate the heat in the area of the bond. Heat a span of about five to seven inches. After a short time the adhesive will soften and the panel can be removed using a panel cutter, chisel, or pry bar. Be careful not to scratch the steel frame as this will remove the corrosion protection.

  9. Some original adhesive may need to be removed from the steel structure to allow space for the replacement bead of adhesive. Care should be taken not to disturb the corrosion protection.

  10. The short section of panel covering the reinforcement needs to be removed. Make a shallow cut the depth of the panel vertically. Heat the outside of the panel along the reinforcement. Once the panel has heated, chisel or pry the panel off. Sand surface to remove excess adhesive.

  11. Verify corrosion protection has been restored. Any scratches to the steel subframe caused by removal of the panels or the structural repairs should have the corrosion protection restored. See question on Corrosion Protection Restoration before proceeding!

  12. The quarter panel is ready to be sized. Cut the panel so a 1/2 inch gap will be left between the panels at each splice.

  13. A section of reinforcement must be removed from the top of the replacement panel to allow proper fit. Cut enough of the reinforcement on the replacement panel so a 1/2 inch gap will remain between the panels when the reinforcements of the panels are butted together. Using the heat gun, heat the reinforcement and remove with a chisel. Whenever a bond is required to be broken, heat the panel or piece that is to be discarded.

  14. Check quarter panel fit. A mill and drill pad system much like that of the Fiero is used for accurate panel placement. Accurate dimensional structural repairs are necessary as only minor inward adjustments can be made by grinding the mill pad.

  15. Backer panels must be prepared. For a rear quarter panel splice, two backers panels are required. The backer panels are fabricated from either the damaged or a scrap of the replacement panel. The backer panels, Photograph 8, should overlap both panel edges at least two inches. 16. Check the fit of the rear pillar backer. Sanding will probably be required before it will fit. Taper all outside edges of the panels at the splice joints to approximately 20 degrees. Make sure to bring the panel edges to a point. This will increase the bonding area for the cosmetic repair material.

  16. Prepare all the bonding surfaces for application of adhesive: backer panels, backside of the panel remaining on vehicle, and the backside of the replacement panel. Clean surfaces with soap and water, and let dry. Sand the bonding surfaces to expose glass fibers. Wipe away any remaining residue with a dry clean cloth.

  17. Apply 1/8 inch layer of Fusor to the top half of the rear pillar backer. Insert the panel behind the pillar. Lightly clamp in place if necessary, leave enough exposed for the required overlap. Wipe off any excess adhesive from the face of the backer. Heat cure the seam.

  18. The next two steps, #20 and #21, may be reversed depending on preference and/or clearances. If bonding the replacement panel on first, make sure to apply adhesive to the upper reinforcement. Photograph 8

  19. Bond in backer at vertical splice. Use the same procedure as with the pillar backer, except apply the adhesive to the forward half of the backer. All backer panels should be in place. Proceed to the next step after all backers have cooled.

  20. Check panel fit one final time before bonding. Place panel on flat horizontal surface. Apply adhesive to the panel, either Ashland or Lord may be used. When applying the adhesive, duplicate the factory bond line of approximately 1/2 inch height and position. Apply plenty of material as this is the only environmental seal. Before the panel is placed into position, place adhesive on the backer panels and reinforcement. Place the panel into position. Fasten the panel to the subframe with the mill and drill pad fasteners. The panel should be resting on the pads with the adhesive filling the gaps. The backers are clamped or screwed to hold them in place. Heat cure all splice joints, approximately 30 to 40 minutes with infrared heat lamp placed 36 inches from the surface. Heat transferred from the joint through the panel will help cure the other bonded areas.

  21. Refer to Scrap Material Available For Backer Panel and Rear Of Panel Accessible Without Removal of Interior Trim. Start at step #13 and proceed to conclusion.

Full Side Panel Replacement (Driver Side)

Full side panel replacement is somewhat overstated because the full panel will not be replaced at the roof. The roof is bonded over the side panels, the replacement panel is sectioned in just above the top edge of the panel at each pillar. This procedure will leave one visible seam at the driver pillar as the other seams are covered by the windows, trim, and the tail lamp.

  1. Remove windows, interior trim, rear fascia, and tail lamp.

  2. Cut panel at each pillar location. Care should be taken not to cut too deeply into the driver side pillar as the door surround panel will be saved.

  3. The bond between the door surround and the panel needs to be separated. Using a heat gun at approximately 400 degrees Fahrenheit heat the panel on the outer edge at the section splice cut. Place a pry bar in the gap between the surround and the side panel. The panel will heat up and the bond will soften, slight pressure should release the bond. Work down the edge to the bottom until the seam is split. Whenever a bond is required to be broken heat the panel or piece that is to be discarded.

  4. The rear of the quarter is covered by the liftgate surround panel. Three options to free the rear edge of the quarter panel are described as follows:

  5. Cut the inner section of the panel and remove. This is done to provide access to the adhesive bondline.

  6. Remove the mill and drill pad fasteners. Four nuts are located along the top edge of the panel. Six screws hold the front and bottom edge of the panel.

  7. The adhesive bond must be broken to remove the remaining panel from the steel structure. Using a heat gun at approximately 400 degrees Fahrenheit, heat the outboard section of the panel to be removed. Concentrate the heat in the area of the bond. Heat a span of about five to seven inches. After a short while the adhesive will soften and the panel can be removed using a panel cutter, chisel, or pry bar. Be careful not to scratch the steel frame as this will remove the corrosion protection.

  8. Sand the door surround panel to remove remaining adhesive and expose fibers. Some original adhesive may need to be removed from the steel structure to allow space for the replacement bead of adhesive. Care should be taken not to disturb the corrosion protection.

  9. Verify corrosion protection has been restored. Any scratches to the steel subframe caused by removal of the panels or the structural repairs must have the corrosion protection restored. See question on Corrosion Protection Restoration before proceeding!

  10. The side panel is ready to be sized. Cut the panel so a 1/2 inch gap will be left between the panels at each pillar splice.

  11. Check side panel fit. A mill and drill pad system much like that of the Fiero is used for accurate panel placement. Accurate dimensional structural repairs are required as only minor inward adjustments can be made by grinding the mill pad.

  12. Three backers panels are required. The backer panels are fabricated from either the damaged panel or a scrap of the replacement panel. The backer panels, Photograph 8, should overlap both panel edges at least two inches.

  13. Check the fit of the pillar backers. Sanding will probably be required before they will fit. Taper all edges of the panels at the splice joints to approximately 20 degrees. Make sure to taper panel edge to a point. This will increase the bonding area for the cosmetic repair material.

  14. Prepare all bonding surfaces for application of adhesive: backer panels, and the backside of the replacement panel. Clean surfaces with soap and water, and let dry. Sand (80 grit) the bonding surfaces to expose glass fibers. Wipe away any remaining residue with a dry clean cloth.

  15. Apply 1/8 inch layer of Fusor to the top halves of the pillar backers. Insert the panels behind the pillar. Lightly clamp in place if necessary, leave enough exposed for the required overlap. Wipe off any excess adhesive from the face of the backer. Heat cure each seam.

  16. Check panel fit before bonding. Place panel on a horizontal surface, apply adhesive to the panel. When applying the adhesive, duplicate the factory bond line by position and height, approximately 1/2 inch. Apply plenty of material as this is the only environmental seal. Before the panel is placed into position, place adhesive on the pillar backers and the door surround. Place the panel into position. Fasten the panel to the subframe with the mill and drill pad fasteners. The panel should be resting on the pads with the adhesive filling the gaps. The backers are clamped or screwed to hold them in place. Wipe off any excess adhesive. Heat cure all splice joints, approximately 30 to 40 minutes with infrared heat lamp placed 36 inches from the panel surface. Heat transferred from the joints through the panel will help cure the other bonded areas.

  17. Refer to previous section Scrap Material Available For Backer Panel and Rear Of Panel Accessible Without Removal of Interior Trim. Start at step #13 and proceed to conclusion.

Door Re-skin

  1. Inspect inner SMC structure for damage. The door is made completely of SMC except for the high strength steel intrusion beam, and steel lock and hinge reinforcements.

  2. Disconnect wiring harnesses and remove door from vehicle.

  3. Cut away the center of the outer panel. Air shears or a pneumatic urethane cutter with a radial saw blade attachment were found to work well because the cut depth is easily controlled.

  4. Remove remaining door skin by heating the bond and applying pressure with pry bar or chisel, Photograph 9. Be careful not to destroy the door flange.

  5. Sand door flange to remove remaining adhesive. Clean bonding areas of replacement panel with soap and water, and allow to dry. Sand bonding surfaces to expose glass fibers. Dry wipe with a clean cloth.

  6. Apply either Ashland or Lord adhesive to the door flange.

  7. Set the panel on the frame and lightly clamp. Be careful not to squeeze too tight as you wish to leave adhesive between the panels.

  8. Allow to cure.

  9. Reassemble and mount on vehicle.

  10. Finish with base/clear coat paint system.

Repair Product Information


The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.

Tech-Cor, LLC, Wheeling, IL
© 1990 Allstate Insurance Company
All rights reserved.