Fillers or Additives are added to resins or epoxt to give certain attributes like strength, smoother application, more bulk, wider bonding matrix or higher density. Each additive has a different task and purpose, so be careful in which additive you are using for what purpose. 

Each additive or filler has its own purpose: 

1- Cabosil is a high density filler used for strength and bonding.

2- Wood Flour is a medium density filler for bonding, laminating, filling and body filling.

3- Glass Bubbles is a low density filler that is used exclusively as a body filler because it can easily be sanded for smooth and stable surfaces before priming and painting.

4- Graphite is added to make a smooth slippery surface that is dense which makes it resistant to scratching and impact damage.

5- Industrial Talc is a bulker that makes resins, polyester resin in particular, thicker when building rounded shapes and flanges. This can be helpful in the mold making process.

6-Chopped or Milled fibers are longer fibers or cotton ball structures that can be used to fill large gaps and can be injected into joints or used to back fill damaged areas.