Is body filler flexible?
Yes, body fillers are flexible. Some body fillers are more flexible than others, but not one body filler is indestructible. The kind of car body filler you use depends on how much flexibility you need for your application; the amount you seek will depend on how thick the layer should be and what material needs to be filled.
Bumper covers are made to be flexible so it is important to choose the right filler for your vehicle.
There are also a lot of body fillers on the market; each has its own flexible properties depending on whether it's designed to adhere well to metal or to be more of a plastic filler. The best products achieve a crack-free finish on the body work and fill holes with ease.
One example of a particularly flexible filler is the Plast X Body Filler from UPOL. It is a high adhesion and flexible polyester filler specifically designed for scuffs and scratches on plastic parts that require ultimate flexibility e.g bumpers. It can be used at all stages of a car's repair, and it's important to ensure the tinier defects (such as small holes) are put right before the final paint coat.
Other flexible fillers for body work include:
Beyond body filler, fiberglass body filler is another option. Fiberglass body filler is a tough and durable material designed for use with fiberglass resin materials such as car body panels (e.g. bumpers), doors, and hoods. It can also be used for applications that require high strength and resistance to impact.
Fiberglass body fillers are made from wet gel coat applied over a fabric mesh reinforcement mat which is then baked in an oven at very high temperatures, resulting in a composite body filler. This body filler is of extremely high tensile strength and also has very good adhesion properties to steel, aluminum, wood, fiberglass body panels, resin body fillers and other substrates where exceptional bond strength is required.
Seam Sealers are another option some consider as a wider variation. Seam sealers are, simply put, a tape that blocks air and water from entering between two panels. They should be flexible to move with the vehicle, but it will eventually crack when exposed to impact or vibration. Seam sealer does not have good adhesion properties so sanding away the seam sealer is recommended before applying a new coat of paint. Seam sealer is a tape, not a filler and they are designed to adhere lightly and can be pulled off without damaging the original surface. Tiger Seal for example works well for plastic issues.