Fumed Silica Filler Details

Fumed silica is a very fine powder that acts as an efficient thickening agent, ideal for use with various resin types such as polyester, vinyl ester, or epoxy. Its primary functions includes creating fillet putty and enhancing the viscosity of resin to improve its performance as an adhesive. When applied to vertical surfaces, resins typically have a tendency to drip or run; however, integrating fumed silica helps prevent this, allowing the resin to maintain its position without sagging.

Additionally, fumed silica offers versatility in applications — it can be combined with milled fibers to produce a robust structural putty or mixed with glass microspheres, also known as glass bubbles, to form a putty that can be easily sanded smooth. A standard mixing recommendation is to add fumed silica to the resin in a one-to-one ratio by volume, although adjustments can be made to achieve the desired consistency specific to the task at hand. Whether you require a thicker blend or a more workable paste, fumed silica provides the flexibility to tailor the mixture to your project's requirements.

Uses for Fumed Silica:

-Boat construction and repair

-Molding, fabrication and repair of fiberglass laminates

-Fillet putty and thickening agent

-Used as an adhesive or putty

-Filling gaps


-Helps prevent sag or run off on vertical surfaces

-Can be used with other fillers such as glass bubbles and/or milled fibers

-Compatible with polyester, vinyl ester and epoxy resins

-Easy application

Needed Supplies When Working with Fillers:



-Fumed silica

-Measuring cups

-Stir Sticks

-Safety Protection including gloves, mask and eye goggles


-Work area and resin should be between 70°F and 85°F for best results.

-Make sure the work area is well ventilated and wear proper safety clothing including gloves and a face mask.

-Measure out the resin in measuring cup

-Add fumed silica until the desired consistency is created. Add a little bit at a time. Fumed silica is very fine and should be added slowly to avoid clumps. stir thoroughly, getting rid of any clumps that may arise.

-Multiple fillers can be added depending on what outcome is trying to be achieved. Glass bubbles are often added as well to help the putty sand easier.

-Add MEKP based on how much resin was used. MEKP catalyst is needed for polyester resin to cure.  It should be added between 1-2% based on the volume of the resin. More or less than this will cause an improper cure.  Cure time and pot life will vary depending on percentage of MEKP added and the temperature of the room.  If 1% MEKP is added, there will be a longer pot life and it will cure slower than if 1.5% or 2% MEKP is added.  Same goes for temperature.  Lower temperatures result in a slower pot life/cure time than if working in higher temperatures.  Fiberglass Warehouse Fiberglass Resin is promoted to gel in 17-23 minutes at 77°F when catalyzed at 1 percent (approx. 1 tsp per pint).

-Apply the resin/fumed silica mixture to surface using a brush, spreader or squeegee

-Uncured polyester resin can be removed from tools and equipment with acetone.

Available Sizes:

4 ounce, 8 ounce, 1 lb. and 10 lbs.

Handling and Storage:

Closed containers should be stored in a cool, dry place.  It should be stored at temperatures below 75°F and away from sunlight and heat sources. 

By Dallin Leach 0 comment


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