There are a variety of fiberglass fillers available on the market. Each one serves a different purpose. One filler can be added or several depending on what outcome is desired. Fillers can reduce or increase weight, improve sanding properties, prevent sag on vertical surfaces, fill gaps in fabric weave, improve bonding properties, add strength etc. It is important to experiment on a small area before using on a project. Fillers are added after the catalyst when used with resins.
Milled fibers come in a variety of lengths such as 1/8″ and 1/4″ (We carry the 1/4" length). They are glass filaments that are coated with a specific sizing to enhance resin compatibility and milled to a specified bulk density. Unlike chopped strands, which are chopped to a precise strand length, milled fibers are milled to an average bulk density, which is determined by the input glass and process conditions. Milled fiber adds strength, dimensional stability, impact resistance and surface hardness. Milled fibers can be added to polyester, vinyl ester and epoxy resins.
Glass Bubbles are another kind of filler. They are hollow glass micro-spheres. This filler is lightweight, easy to sand and can be used on minor surface repairs. It makes a good putty for cracks and holes when mixed with resin.
Fumed Silica (commonly referred to as Cabosil or Aerosil) can be used with polyester resin, epoxy resin and urethanes as a thickening and thixotropic agent. It helps prevent sag or run off on vertical surfaces. Fumed silica may be used with other fillers such as glass bubbles and/or milled fibers to achieve additional structural properties. It is ideally used as an adhesive or putty. A downside to fumed silica is that it is difficult to sand.
Phenolic microspheres are purple/brown microscopic spheres. They are easy to sand and great for light weight compounds or for filling large holes. Do not use this filler when making compounds intended for gluing. Phenolic microspheres will no crush like glass bubbles, but will flatten out.
Use safety precautions when working with fillers and resin. Make sure to protect your eyes and skin.