Modifying with Fillers & Additives


Throughout this and other WEST SYSTEM manuals, we refer to epoxy, neat epoxy or resin/hardener mixture, meaning mixed resin and hardener without fillers added; and thickened mixture or thickened epoxy, meaning mixed resin and hardener with fillers added. Fillers are used to thicken epoxy for specific applications such as bonding or fairing.

After selecting an appropriate filler for your job (see Filler Selection Guide), use it to thicken the epoxy mixture to the desired consistency. The thickness of a mixture required for a particular job is controlled by the amount of filler added. There is no strict formula or measuring involved-use your eye to judge what consistency will work best. The chart below gives you a general guide to the differences between neat (unthickened) epoxy and the three consistencies referred to in this manual.

Always add fillers in a two-step process:

  1. Mix the desired quantity of epoxy resin and hardener thoroughly before adding fillers. Begin with a small batch-allow room for the filler.
  2. Blend in small handfuls or scoops of the appropriate filler until the desired consistency is reached (see chart below).

For maximum strength, add only enough filler to completely bridge gaps between surface without sagging or running out of the joint or gap. A small amount should squeeze out of joints when clamped. For thick mixtures, don’t fill the mixing cup more than 1/3 full of epoxy before adding filler. When making epoxy fairing compounds, stir in as much 407 Low-Density Filler or 410 Microlight Filler as you can blend in smoothly—for easy sanding, the thicker the better. Be sure all of the filler is thoroughly blended before the mixture is applied. Spread the epoxy mixture into a thinner layer, either around the inside of the mixing cup or onto a flat non-porous surface or palette, to extend its working life.

Epoxy can be thickened to the ideal consistency needed for a particular job. The procedures in this manual refer to four common consistencies: syrup, catsup, mayonnaise and peanut butter.


 syrup  CONSISTENCY: Unthickened epoxy mixture
 CHARACTERISTICS: Drips off vertical surfaces
 USES: Coating; wetting-out before bonding; applying fiberglass, graphite and other fabrics


CONSISTENCY: Slightly thickened epoxy
 CHARACTERISTICS: Sags down vertical surfaces
 USES: Laminating/bonding flat panels with large surface areas, injecting with a syringe


 mayonnaise  CONSISTENCY: Moderately thickened epoxy
 CHARACTERISTICS: Clings to vertical surfaces; peaks fall over
 USES: General bonding, filleting, hardware bonding


 peanutbutter  CONSISTENCY: Maximum thickness epoxy
 CHARACTERISTICS:    up  Clings to vertical surfaces; peaks stand
 USES:  Gap filling, filleting, fairing, bonding uneven surfaces



Additives are used to give epoxy additional physical properties when used as a coating. Although additives are blended with mixed epoxy in the same two-step process as fillers, they are not designed to thicken the epoxy (see Additives & Pigments). Follow the mixing instructions on the individual additive containers.