The SMC hood was ground in preparation of work, and then I used epoxy and fiberglass to complete the work and restore the structure and shape back to original for this truck hood.

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The following is how I did it.

Composite Grinding Tools

Beginning with personal safety, I used a dust mask to prevent inhaling the dust into my lungs.  I used OSHA Z87 approved safety glasses to protect my eyes.  A pair of earplugs protected my ears.  Abrasion/dust resistant gloves for my hands, and a light jacket to keep the dust from my skin.

For removing the layers of SMC to feather edge the surfaces, I used several abrasive tools.  A five-inch pneumatic grinder with 80 grit sandpaper worked well on the large areas to quickly remove large amounts of material.  A right-angle die grinder with Roloc 3″ or 1.5″ sanding disks worked well for the concave areas and other difficult access areas.  A straight die grinder with a fluted burr worked to get into the detail areas of the front grill and other cracks that needed material removed.

All of the tools I used were air powered, so I had a good air supply that would keep up with me.  The large amount of dust that is produced from this sort of work presents explosion hazards when using electric tools.  Large dust piles can also spontaneously combust, so care must be taken during dust storage and disposal.

A air blow gun was also used in this process to clean the dust from the part and from my clothes.  It is important to keep personal safety equipment such as safety glasses and earplugs in place when using the blowgun.  A small vacuum can also be substituted with care taken to prevent dust ignition/explosion.

After all of the material is removed from the SMC hood, all of the dust is blown off to leave a part with a bonding surface that is clean, dust-free and ready to be repaired with epoxy and fiberglass.

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Time, effort, epoxy, and fiberglass, allowed me to successfully completed the repair.  This SMC hood will be repaired as good as new!

Damaged area to be repaired

The hood was placed upside to allow for work access.  The repair area had several cracks and breakage areas.  The repair began with surface preparation.

Surface Preparation on the repair area

I used a grinding disc to remove material on the front and back side of the repair to scarf the repair into the large area.  Good surface preparation gives us a clean area that can hold a bond and create a transition area.  I also drilled some holes in the end of the cracks to stop their propagation.

The back side was prepared too

The back side was prepared for repair by grinding the surface to allow for good bonding by the epoxy resin and glass reinforcement.

Beginning to add material

I started to add epoxy and fiberglass back onto the fender.  I had to gradually move back out to where the existing fender used to be.  I worked both sides bit by bit and allowed it to start curing before adding more.

Getting the shape back

I actually went a bit beyond the shape I needed.  This allowed me to get the surface planes in the right spot, and later return and trim back the proper hood edge.

Back side getting rebuilt

The back side of the repair also got transitioned into the hood and out to meet the front repair area.

Grinding the repair

After most of the rebuilding was complete, some grinding got it back into shape to allow an evaluation of the repair progress.

Grinding on the back

The back side was cleaned up as well, getting the appropriate part thickness back to where it was originally intended.  A nice transitioned surface was created, and all sharp edges were removed.

Bodyfiller for cosmetics

I used some bodyfiller to smooth the repair and restore the cosmetics of the hood.  I could also have used epoxy, and it would have bonded better.  Polyester bodyfiller is less expensive, easier to apply, and easier to sand.  I smoothed the surface and feathered it back into the surrounding area to allow for a consistant surface.

Application of Primer-Surfacer

Urethane Primer-Surfacer is applied to the whole area to allow for removing the sanding/grinding scratches and preparing the surface for paint application.

The repair is complete, and the hood looks back like it was original.  This repair, and many others, can be detected with some investigation of the back side of the repair area.  This will be a very durable repair and will last as long as the rest of the composite body panel.

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The advantages of using composites in this application include non-conductivity, protection from impact, and the ability to resist corrosion in tough environments.  This is why it has outlived it’s factory setting.

Composites enclosure

An item like this where it has a good surface finish on both sides is going to be made in a set of matched-mold dies in a high-pressure and high-temperature press.  It is likely to be manufactured with the Sheet Molding Compound (SMC) process, though Bulk Molding Compound (BMC) could also be at work.

Electrical panel enclosure

You can see on the front door all of the holes from its former installation.  Holes on the side were used as well.  It was customized for its application with all of the holes, and a new unit would arrive “blank,” without holes, as most applications are customized.  The composite can be cut and drilled with metal working tools such as drill bits, holesaws, and saw blades.

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The Big Hole

The picture is showing the hood upside-down on a work table.  The hole is the result of some sort of long-ago impact the shattered the composite material and did significant damage.  My first reaction was to just repair it from the backside.  This hope was lost when I quickly discovered that there was not any access to the backside of the repair.  At least not without cutting some support structures out of the way.  Which would mean that I would need to rebuild those after I repaired the hole.  Not impossible, but it seemed like a lot of work.

I began this repair like any other, with the surface preparation.  The hole got larger as I removed all of the damaged material and created a transition area.

Damage is removed and transition area prepared

The edge of the hole was a razor’s edge as I transitioned out to the existing finished surface.  I also used some sandpaper to sand around the inside of the hole to allow epoxy to adhere to the inside surface when I put in my patch.

My strategy was to create a thin layer of composite using epoxy resin and 3oz Chopped Strand Mat.  The layup was done on a piece of plastic and allowed to cure.  Once cured, I cut it to be about 1/2 inch larger than the hole.  Then I mixed up some thickened epoxy and applied it around the perimeter of my patch and placed it in the hole.  I had already placed a sheet metal screw in my patch, and attached a wire to a support to hold the patch in place until cure.

Patch in the hole

In the picture you can see the patch, the wire, and the support.  The tension on the wire held the patch in place until the patch was cured in place.  Next I worked to add material from the front to fill in the low spot.  This returned strength to the laminate and restored the surface profile back to where it should be.  I again used 3oz Chopped Strand Mat and started with small diameters and worked out to larger ones until the surface at the correct level.

Most of the Glass and Resin have been applied

Now most of the surface has been filled in, and it can be ground to remove any high spots, air bubbles, and allow for adhesion of the next layer.

Grinding the surface

Now I can fill the low spots with thickened epoxy or bondo and sand the surface to match the surrounding areas and get a nice finish.

This is how a repair should be made.  A nice transition in the existing laminate will keep the repair from cracking or breaking in the future.  After the bodywork is complete nobody will know it was repaired.

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Chopped Strand Mat

The chopped strand mat is made up of random glass fibers held together with a light binding material.  A good pair of scissors can cut the glass into manageable sizes, though the scissors will be dull when you are finished.  The chopped strand mat can also be torn by hand.  This leaves a “feathered” edge to create nice transitions between the patched areas and the existing surface.  Pieces that are cut will leave a silhouette that will show and may require other methods to make it disappear.

The first thing that I checked was to make sure the glass was compatible with the epoxy resin.  The package told me so, but I also did a small test sample to ensure that the epoxy would mix with it, was workable, and would harden properly.  Testing away from the actual part can save many potential headaches and pitfalls.

Applying the glass and resin is relatively easy.  Thoroughly mix some resin and apply it to the properly prepared surface (clean, dry, and sanded).  Tear off an appropriate size piece of glass and place it on a piece of cardboard.  Use a cheap paintbrush to get the glass wet with resin.  Flip the glass over, and wet the backside.  The glass will go from white to transparent on the cardboard.  Now lift the glass from the cardboard and apply it to the area with the resin recently-applied resin.  Use the brush to push it down against the surface to get as much contact area as possible.

Additional layers can be applied on top to build thickness, making sure to create a nice transition and limiting air bubbles in the laminate.

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Fender corner damaged by cracks

I began the work by preparing the area by grinding away the outside layer of existing material.  This helps us get a good mechanical bond and a clean surface.  It also allows for a feather edge transition between the repair area and the existing composite.

Fender getting epoxy and glass

The ground-out area is within the black outline created by a primer layer, and the underlying SMC is white in color.  This area was ground with 40 grit sandpaper and followed by a healthy dose of compressed air to remove the dust from the process.  I then wet-coated the areas with epoxy resin before turning to the glass and wetting that out.  The soaked glass was applied to the back side of the fender to add strength back to the cracked laminate.  A thickened epoxy paste was added to the front to fill the cracks that were ground out, as well as filling the holes drilled in the end of them.

Driver's fender from the backside

The glass applied to the area was applied down with a strong bristle brush to get maximum surface contact and remove air bubbles between the layers.  The strength of the glass and epoxy will restore this area of the hood back to its original specification as long as a good bond is made to the composite laminate.

Trimmed after cure

As the resin and glass cure, there is a time window when the resin is in the solid phase, but not rock hard.  This is when it is appropriate to take a utility knife and trim the edge of the loose glass.  If it is tried too soon, it will move the glass on the laminate repair, and if it is too late, the knife will not cut through.  If this time frame is passed, it still can be cut with a saw blade or grinder, but is much messier.  This is also a good time to wipe up any resin that is draining from the repair.

The repair area can now be sanded and body finished before paint and primer!  This resin will need to be covered to prevent UV damage in the future.

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SMC Truck Fender Extension

This SMC part is all one single molded piece, having very complicated geometry.  It can also be noted that there are not any undercuts, i.e. the mold can open and close without having to move around the part. It has a mostly constant cross section, is not supported by any metal struts, and is attached to the cab by three bolts.  The geometry and details to match the cab and hood styling are molded-in so that the part can be painted and bolted on the truck.

If a part needing this much complexity was manufactured with sheetmetal, it would have many more pieces of the assembly and much more bracing.  Metal would also not have the resistance to corrosion and impact that is enjoyed by this composite part.

This fender extension is from a truck that has been on the road since 1993, with over 270,000 miles on it.  This part has been in the wild for over 15 years, and has been exposed to tons of road salt, debris from the tires, and lots of other environmental exposures.  It has survived well, only needing an update in paint to refresh its look.

Another great application of composites!

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The main goal of this substitution is to reduce weight, which improves fuel efficiency.  Other positive side effects include fewer assembly operations, additional design freedom, dent and impact resistance, and the elimination of corrosion.  Several “green” resin formulations have been introduced that make use of bio resins, which use much more renewable resources such as soy products.  The fillers and reinforcements in this material can also be made from recycled and renewable materials.

SMC has overcome several hurdles in order to get to its present use and application.  General acceptance and education had to be proven to the OEM manufacturers and consumers.  Paint application and adhesion was one large consideration that had to be proven out.  There were issues with popping and blistering from the SMC surface.  Making sure the SMC parts held dimensions and aesthetics was also an important milestone.

SMC has become widely used today for many applications, and will find its way into many more.  The weight saving aspects are paramount for reducing fuel consumption.  The anti-corrosion and dent resistance are loved by consumers.

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