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December 30, 2009 4 min read

Epoxies formulated for coating and fiberglassing are too thin to serve as gap-filling adhesives. They can be modified by the addition of thixotropes to form non-sagging pastes very useful as gap filling glues. These pastes can be further modified with the addition of microballoons to form putties for fairing and hole filling. Wood flour can be used to make filleting putty for stitch-and-glue boat construction. All these solid dusty additives are called fillers. Fillers change the flow and density characteristics of the epoxy system. Each filler changes the liquid resin and hardener in ways that make epoxy useful for other applications besides coating or fiberglassing.

You will need to learn how to use fillers if you intend to use our general-purpose epoxy. You can avoid using fillers if you choose to use products from the SilverTip Series. SilverTip GelMagic, SilverTip EZ-Fillet, SilverTip QuikFair, and SilverTip MetlWeld have already been modified and optimized for their particular end uses. You simply measure and mix these products and do not have to bother with fillers. So, if you are using any of the SilverTip Series products, skip this and move on to the next section.

Fillers fall into four general classes: thixotropic agents, bulking agents, fibrous fillers, and pigments. There is some overlapping as to function of certain fillers. For example, plastic minifibers are both fibrous and act also as a thixotropic agent.

Silica thickener (Cab-O-Sil or Aerosil), plastic minifibers and wood flour are thixotropic agents. They turn the epoxy into a thixotropic fluid. These fluids flow under shear stress but do not readily flow once the stress is removed. Ketchup and latex house paints are examples of thixotropic fluids. Adding these agents to the mixed resin and hardener produces a fluid that will easily flow under the spreading stress of a putty knife. Once the stress is removed the thickened epoxy retains its shape. In short, these fillers make the epoxy non-sagging and are added specifically to make gap-filling adhesives.

Phenolic microballoons, quartz microspheres, and wood flour are bulking agents. They "bulk out" the epoxy making a light weight putty like mix. Although all these thicken the epoxy, only wood flour will also make it thixotropic. Attempting to add sufficient microballoons or microspheres to make a non-sagging fairing putty will result in one that spreads poorly as it becomes dry. These materials should be used along with a thixotropic agent. Silica thickener is the best choice because it produces the smoothest compound.

Chopped glass strands, milled glass fibers, and plastic minifibers are fibrous materials that can be incorporated into structural filleting putties to improve tensile strength, and are listed above in descending order of tensile strength improvement.

White paste pigment (titanium dioxide) and graphite powder are generally used as pigments. Graphite powder added at high loading levels (25%) to coatings, which are then sanded produce a "slick" racing finish due to the lubricating qualities of the graphite. Graphite is a conductive material and could cause electrolysis problems under the right circumstances. Since it is the most "noble" of all conductors you should avoid direct contact with other metals under wet conditions. Adding white paste pigment produces a white resin coating that is useful where a light color is desired and painting is difficult. Pigments aren't meant to serve as substitutes for paint in areas exposed to strong sunlight. White paste pigment can be added to the final fill coat when fiberglassing, allowing this coat to serve as a base coat for finish painting.

Our other pigments are pure dry colorants ground into epoxy resin to produce an epoxy paste pigment. Since they are dispersed into epoxy resin they may be added to the resin side of our epoxy systems to produce stable pigmented resin. The volume of the pigmented resin is used to determine the hardener necessary. These pigments are transparent when used in tiny amounts in an epoxy and can be said to act as dyes. In larger amounts they are opaque. Our pigments come in white, black, brown, yellow, red, green and blue and may be blended with each other to produce various hues. They should be used in epoxy systems only and never used in our paints.

These fillers, pigments and additives may be used with any of our epoxy systems except for SilverTip Laminating Epoxy, which was designed as a coating and fiberglassing resin only. Higher filler loading levels are possible with Clear Coat epoxy because it is much lower in viscosity than our other systems.

Fillers change the mechanical properties of the cured resin. For all practical purposes the builder can ignore these changes. Thixotropic agents have the least effect since they are used in the smallest amounts to produce the desired result. Bulking agents reduce tensile strength in proportion to the amount added. Some will initially increase compressive strength. With increasing amounts of additives, though, compressive strengths will decrease.

Many combinations of filler materials are possible and we have not tested them all. If you have an idea that a certain combination might do something special for you then check it out. Little pieces of scrap plywood are good for this. Think up some destructive tests that will simulate the stresses the material will see in service. Check to see where the failure occurs. If the wood breaks then your combination should work well with wood, at least.

The correct sequence for the addition of filler materials:

  1. Correctly measure and mix resin/hardener.
  2. Add fiber fillers, if any, and mix well.
  3. Add bulking agents, if any, and mix well.
  4. Add thixotropic agent and mix well.

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