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–Driver’s Fender Corner
One of my recent projects was the repair of a large SMC fiberglass truck hood. It required several areas of repair, one of them including an outside wheel opening on the driver’s side of the hood. This area had a couple of large cracks from some sort of impact trauma.
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.
RV and Specialty Vehicle Market
One of the industries that makes heavy use of fiberglass parts is in the RV and specialty vehicle market. Mostly centered around Elkhart, Indiana, the manufacturers use Fiberglass in many form and function areas of their vehicles and trailers. These RVs, trailers, and specialty vehicles range from small, single axle campers to trailers pulled by semis, and from vans to tandem axle buses. Specialty vehicles include ambulances, firetrucks, handicapped vans, and commuter vans.
The advantages of fiberglass composites mesh very well with their use in various applications in this industry. The strength-to-weight ratio is very important, along with the good adhesion of automotive paint. The low tooling cost of fiberglass relative to sheetmetal is a huge advantage, allowing for inexpensive low-volume production. The long service life and resistance to corrosion is another advantage over competing materials.
There are a few disadvantages for composites. Cracking can develop over time with improperly supported structures. Depending on processing, the smoothness of the surface (surface profile) can be subpar to that of sheetmetal, and is subject to worsen over the first year of its life. Paint adhesion can be a problem for all materials, and fiberglass composites have their own unique issues.
With proper processing, all of the disadvantages can be overcome. A properly designed and supported structure with good workmanship will never crack and will last forever. Surface profiling can be eliminated with a good print blocking material, good skin coat, and good resin. Paint adhesion can be optimized with the correct gelcoat and paint preparation methods and materials.
Fiberglass composites are well utilized in the RV and specialty vehicle market and will remain there forever. They are the best solution for the many challenges faced by the folks that build these great RVs.
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