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Epoxy Additives, Fillers, and Thickeners FAQs

Why are additives used with epoxy?

Adding thickeners and fillers to epoxy will change its physical properties, sometimes drastically. For example, without additives, epoxy often runs and sags when applied to overhead and vertical surfaces.

What do epoxy additives or fillers do?

By blending in a specific additive or filler, and mixing it to the desired consistency, the epoxy can become suitable for varied applications. Examples include fairing, bonding, and filling voids. Additives also help to prevent epoxy from sagging or dripping when applied to a ceiling or a vertical surface.

What are the different consistencies of resin and what are they used for?

Epoxy is typically thickened to 4 different consistencies to suit a wide range of applications. Syrup, or un-thickened epoxy resin without fillers, is a runny liquid consistency that will drip off a vertical surface. This is often used for laminating composite fabrics like fiberglass cloth, or for wetting out before joint bonding or coating tasks.

Slightly thickened epoxy has a Ketchup consistency, containing a tiny bit of filler along with blended resin and hardener. It will still sag if applied to a vertical surface. Ketchup-like epoxy resin will easily laminate and bond flat panels with generous surface areas. Or, use a syringe to inject this barely thickened liquid into a project.

Next, epoxy with the consistency of Mayonnaise is a moderately thick blend of resin with fillers. It will cling to a vertical surface, and is handy for filleting and bonding tasks.

A blend with the consistency of Peanut Butter epoxy resin easily remains on a vertical surface. If you pull the epoxy mixture into peaks, they will hold their shape. This consistency is best for filling gaps, fairing, and bonding uneven surfaces. It creates a nicely shaped fillet joint.

What can you use to thicken epoxy? How do you select the right epoxy filler?

Choosing the right epoxy resin thickener or filler is key to achieving your desired result. Instead of eyeballing the additive amount, follow the manufacturers instructions to achieve the right consistency.

Cabosil & Colloidal Silica

This epoxy thickener changes resins viscosity and bond strength. Cabosil-thickened resin, or cabosil filler, is useful for bonding, gap filling, and reducing sags or runs.

What is cabosil / colloidal silica used with epoxy for?

This epoxy thickener controls the epoxys viscosity, and creates a smooth blend that helps to prevent runs and sags on overhead and vertical joints. This multi-purpose additive is useful for epoxy filleting and bonding. It is also used to increase epoxy fairing compounds consistency, strength, and abrasion resistance. Silica epoxy thickener can also be used with other fillers.

Milled Glass Fibers

This finely ground fiberglass additive increases tensile strength and boosts a layups integrity. Add the milled glass fibers to polyester resins, epoxy/hardener systems, and structural filleting putty. To prevent draining and sagging, add silica thickener aka cabosil to the enhanced resin.


This finely textured fiber blend will thicken resin and hardener, and helps to form an adhesive especially suited for bonding wood. Microfibers create a compound with good gap-filling characteristics without compromising its wetting and penetrating attributes.


Inexpensive low-density filler made from sodium borosilicate. When combined with Silica thickener, they help prevent sagging. Tiny (actually microscopic) microballoons are among the most versatile epoxy resin additives. Also called microspheres, these hollow glass structures are designed to be mixed into polyester resins and epoxy systems.

Lightweight, low-cost microballoons help to thicken and extend the epoxy resin. Microballoons also help to make an easily sanded fairing compound or non-structural fillets.

Acrylic microspheres weigh even less than glass, and they are cheaper than phenolic microspheres. The acrylic microspheres can be affected by some solvents or by heat, although that does not seem to be a major problem.

Wood Flour

Made of finely cut sawdust, brown-colored wood flour is ideal for making structural fillets and gluing joints. Also an effective gap filler. Great for naturally finished wood, and requires little sanding.

What is the difference between microballoons and low-density epoxy filler?

Microballoons and low-density epoxy filler share two similarities. When added to epoxy, both substances help to create a viable fairing compound that is also somewhat easy to sand. However, these fillers differ in one major aspect. Where tiny microballoons help to create a lighter-textured fairing compound, the low-density filler blends into a substantial fairing putty with a peanut butter-y consistency. Besides sanding this fairing putty, the user can also carve it once the substance has cured.