What is the appropriate body filler? 

Cosmetic repair today has many options for repair materials. During the collision repair process, a technician is faced with what is the appropriate body filler. There are various options of body filler that have been introduced over the years, all of which have different characteristics and applications. 

New types include lightweight polyester body fillers, polyester glazing putty, fibreglass-reinforced body fillers, metal body fillers, sprayable polyester body fillers, and specialty body fillers.  

Lightweight polyester body fillers 

For non-plastic repairs, lightweight polyester body fillers are the most used cosmetic repair material. This type of body filler is easily sanded and spread and can be used for general purposes or premium performance products.  

Polyester glazing putty body fillers 

Polyester glazing putty is a thin-bodied polyester filler designed for minor fillings, such as filling pinholes and skim coating. These body fillers sand easier with finer grit sandpaper. In some instances, this can even be applied over cured paint that has been prepared properly according to the product maker.  

Fibreglass-reinforced body fillers 

Fibreglass-reinforced body fillers are polyester body fillers with either short or long fibreglass strands for added strength. It is most commonly used over welded seams or for repairing sheet-molded compound (SMC) or fibreglass. Most are waterproof; however, this should be confirmed with technical data sheets (TDS). Fibreglass-reinforced body fillers should be finished with a polyester glazing putty before applying any primer. 

Metal body fillers 

Metal body fillers add aluminum to the body filler to increase strength, durability and resistance to water. Some metal body fillers allow drilling and tapping. In some cases, original equipment manufacturers (OEMs) require this type of body filler for welded seams. A key difference with this filler compared to others is that its uses a liquid hardener instead of a cream hardener. 

Sprayable polyester body fillers 

This type of body filler is a thin polyester that is applied by a large-tipped spray gun. It enables a high film-build and uniform surface over large repair areas. When using sprayable polyester body filler, always refer to the TDS for proper mixing and application recommendations. 

Specialty body fillers 

Specialty body fillers are created to solve specific problems. These body fillers offer certain characteristics such as flexibility, increased strength, faster cure times, or better adhesions to certain substrates. Some examples of specialty body fillers include flexible polyester body filler for plastics, carbon fibre-reinforced filler for SMC or fibreglass, or spot putty for fast filling of pinholes prior to refinishing. 

Some unique body fillers have come to market, including: 

• Colour changing body fillers 

• UV-cured body fillers 

• Slow cream hardener  

These new products have become a significant aid for refinishing technicians to achieve optimal results. For instance, using a colour-changing body fillers ensures that the filler is ready to sand, and a slow-cream hardener enables a shop to be more efficient in hot weather and on large repairs. 

Proper preparation leads to best results 

Proper preparation is required for the body filler to adhere to steel, aluminum, fibreglass, SMC, and OEM paint. Many body fillers can be applied over epoxy primer, but compatibility must be confirmed by the paint maker and specific OEM recommendations should also be considered.  

To achieve optimal results when using body fillers, it is imperative to always reference the TDS for product-specific recommendations. However, it is equally as important to consider the following: 

• Check expiration dates of body filler and hardener. 

• Use the same brand of hardener and body filler. 

• Clean. 

• Featheredge the repair area out far enough. 

• Apply within the substrate temperature range. 

• Use accurate mixing ratios, which could be done by weight or using a system that mixes the two together inside a nozzle. 

• When mixing, fold the body filler—do not stir it. 

• Apply multiple thin layers rather than one thick coat of body filler. 

• Do not exceed the maximum body filler thickness after sanding. 

While body fillers may seem trivial, it is critical to follow the body filler and paint TDS, as well as OEM recommendations. 

To learn more about appropriate body fillers and their different uses and applications, consult the I-CAR Canada catalogue on refinishing courses.