Auto body filler hasn't always had a great reputation. It has been known to crack, chip, and require several hours of extra labor if it's not applied perfectly. After hearing that your technician will be using filler to repair your vehicle's body damage, you may be skeptical of the finished product. However, with automotive body metals and materials becoming lighter and thinner over the past several decades, body filler has become essential to nearly every collision or dent repair job. Here are the advantages and disadvantages of the three primary types of filler:
Premium body filler is made with resin and powder. The primary advantage of using premium filler over short strand or long strand is that it's extremely easy to sand—if it's sanded at the right time. Premium filler requires the use of a catalyst that drastically decreases the cure time of the filler. For this reason, premium filler must be mixed, applied, and sanded within about 15 minutes (depending on the amount of catalyst used).
Due to the cure time and ease of use that comes with premium filler, body repair jobs that call for premium filler will typically have a significantly decreased labor cost when compared to jobs that require short or long strand filler. Due to the smoothness of premium filler, some jobs that call for premium filler won't even require an additional coating of finishing putty—which further reduces labor and supply costs.
However, the main issue with premium filler is that it's brittle. As your vehicle's metal body heats or cools in varying weather conditions, it will slightly expand and contract. Since premium filler is only held together with powder, it won't flex along with your fenders, quarter panels, or bumpers. Instead, it can crack and ruin the upper layers of your vehicle's body.
For these reasons, premium filler can only be applied in thin layers. As long as the filler is thin, it's able to tolerate the expansion, contraction, and vibrations your vehicle experiences on a daily basis.
Short Strand Fiberglass
Short strand filler contains fiberglass strands that provide additional strength to your vehicle's damaged body. Strand-type fillers are drastically different from premium filler because they consist primarily of resin instead of powder. As a result, fillers that contain strands of fiberglass are more dense and durable than premium filler.
Short strand filler (also referred to as chopped strand) contains very short lengths of fiberglass. These short strands allow the filler to be molded to the curves, dents, and uneven surfaces found in your vehicle's body while still providing reinforcement to heavily damaged areas. Since short strand filler isn't completely powder-based like premium filler (though it does contain some talc), it's typically used in areas that must be protected against water penetration.
Adhesion is one of the main benefits of short strand filler. Short strand filler provides the most adhesion out of the main types of body fillers. For this reason, it's also used as a sub-layer for other fillers. Your auto body technician can apply a layer of short strand filler to a gash or dent, and cover the short strand filler with an additional layer of premium filler to smooth the area.
Long Strand Fiberglass
Similarly to short strand, long strand fiberglass filler is used in areas of your vehicle's body that require significant reinforcement. Large gashes, punctures, and other similar types of piercing damage will typically require the use of long strand filler.
Long strand filler doesn't provide as much adhesion as short strand filler, but it provides something else: flexibility. The adhesion and flexibility provided by long strands of fiberglass allow long strand filler to reinforce large sections of a damaged body part.
However, due to their hardness after curing, both short and long strand fiberglass fillers require a significant amount of sanding. If your technician must use either of these types of fillers, then expect to have additional labor costs associated with your repair. Sanding fiberglass fillers not only requires a strong hand, but the use of several sanding blocks and tools as well.
As you can see, each of these three types of fillers must be used under difference circumstances. When your vehicle is inspected by your local auto body shop, make sure you know what type of filler will be used to fix your body damage. By knowing the pros and cons of the filler used on your vehicle, you will have an understanding of both the quality of the repairs and the work that went into them.