Repairing the Inner Fender

One of my recent projects involved the repair of a 1993 International Medium-Duty truck hood made from SMC.  There were several areas needing attention, and one of them was the driver’s side inner fender.  This piece had formerly been attached with button-head pop rivets.  This design is common to composites, and allows for easy replacement of the separate fiberglass pieces.  The pop rivets had come loose over time, allowed to move around, and cause severe damage to the extent that the riveting flange was broken off.  My only solution was to bond the two pieces together.

Material Fatigue in the corner

The loose panel flexed so much and for so long that it fatigued the material and failed in the corner of the inner fender next to the attachment to the rest of the hood.  To repair this, I removed the area with the rivets, ground down the surfaces of both pieces on both sides, and reattached them with fiberglass and epoxy resin.

Prepared glass and resin

I wanted to place epoxy and fiberglass on both sides of the repair area to ensure a good, solid bond that would hold very well.

Epoxy Resin and Fiberglass applied

After the area was prepared, I applied epoxy resin to the surface to ensure good adhesion.  I had a low spot that was a gap, so I mixed some microfiber and epoxy to make a paste and fill this gap.  A stronger bond is produced when the fiberglass is not spanning an open gap between the two pieces.  I placed two layers of 3oz Chopped Strand Mat over the paste and worked the air out to make a nice consistent repair.  I then ground down the surface to make a nice-looking, consistent repair.

The rear of the inner fender had similar problems.  A hole had emerged in the black SMC piece.  I ground down both surfaces and placed some fiberglass across the area to bond it together.

IH Hood Repair– Passenger Corner

One of the repairs on my International Hood was for the passenger corner.  This portion of the hood was damaged before I got it, and was COMPLETELY MISSING!

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.

Working with Bondo

Commonly referred to by the trade name of “Bondo,” polyester body filler is used by autobody shops to achieve cosmetic repairs on automobiles.  It is relatively inexpensive, easy to work with, and achieves good results when used properly.  It is basically polyester resin that is highly filled to create a thick paste which can be turned into a solid with the addtion of a hardener.

I have been using autobody filler since age 14, and am finally getting good at it!  Just kidding.  It is more art than science, and practice makes perfect.  Basically, a surface needs to be properly prepared to achieve good adhesion.  Then the filler is applied after being mixed with hardener, and allowed to cure afterwords.  Some fillers can be sanded as soon as 20 minutes after application.

Basic setup and tools

The basic setup is a working board surface to mix the bondo and hardener.  The mixed material is applied with a squeegee or putty knife to fill in low areas.  They also aid in cleanup, along with some paint thinner to clean the tools.

Hardener added

Mixing the hardener starts the application window from where the material goes from liquid to solid.  The hardener is a peroxide paste that directly affects cure time along with temperature.  Too little hardener can also cause poor physical strength characteristics along with difficult sanding properties.

Uniformly mixed

The hardener must be uniformly mixed before it is applied.  This prevents lots of problems down the road.

Applied to surface

The body filler can be applied with putty knives or squeegees to achieve the desired affect.  This polyester bodyfiller will shrink during cure, so filling high is typical.  The flip side is that all of the high spots must be sanded back down, which is wasting filler and time, so it is a fine line to walk.

Working with body filler can be frustrating for the beginner who must practice lots of patience.  There are many important aspects that must be followed, just like all composites materials processes.  Autobody filler requires proper surface preparation, mixing and metering of materials, and attention to details in order to avoid future problems.

A couple of other practices I recommend is to always wear a dust mask, keep the body filler dry, and only apply bondo over sanded bondo.  Some like to just build layers without sanding the lows, and this can lead to adhesion problems.

Hood Repair– A big hole!

My IH SMC truck hood project involved repairing a large hole.  This was one of my largest hurdles to the whole project.  This is what I initially saw.

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.

Working with Chopped Strand Mat

One of my projects is repairing a composite hood for a large International Medium-Duty truck.  This hood is made of SMC, and is damaged in various locations to various degrees.  In doing the repairs, I am using epoxy resin and glass reinforcement.  The fiberglass reinforcement I am using is referred to as Chopped Strand Mat, and is the 3 oz. per square foot version.

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.