How to Mix Fiberglass Resin & Cab-o-Sil

Written by alex smith
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How to Mix Fiberglass Resin & Cab-o-Sil
Auto body repair can be aided with fibreglass resin thickened with cab-o-sil. (a car in a car show image by Gary from

Cab-o-sil belongs to a range of products called fumed silicas. These products are essentially powdered glass and are used to thicken liquids such as fibreglass resin. An alternative to fibreglass gelcoat can be created by mixing resin with cab-o-sil, creating a "mud" that has the same properties of commercial gelcoat at a much lower cost. Special attention must be paid to safety when using cab-o-sil, as the powder can easily coat your lungs and cause serious health problems.

Skill level:

Things you need

  • Fibreglass resin
  • Cab-o-sil
  • Mixing stick
  • Bucket
  • Cup
  • Rubber or vinyl gloves
  • Respirator

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  1. 1

    Put on a respirator and protective gloves.

  2. 2

    Pour fibreglass resin into a bucket. It is best to use more than you think you will need, as it is very difficult to make a second batch that is the exact thickness as the first.

  3. 3

    Measure an equal volume of cab-o-sil into a cup. Set it aside.

  4. 4

    Add the catalyst to the resin, following the instructions on the product.

  5. 5

    Stir in the cab-o-sil a little at a time. The cab-o-sil will naturally sit on the surface of the resin and puff away if you stir it too forcefully, so you must first cover it with resin using the mixing stick. Once the cab-o-sil has been saturated with resin, you can stir normally. All of this must be done quickly as the resin is already catalysing.

  6. 6

    Use the thickened resin as needed for auto or boat repair.

Tips and warnings

  • Some people prefer to make large batches of cab-o-sil and uncatalyzed resin, which can be stored and used as needed. If you choose to use this option, be aware that it can be difficult to get the catalyst to mix thoroughly in resin that has already been thickened.
  • Always work with proper ventilation and wear a respirator when using cab-o-sil.
  • Wear protective gloves when using fibreglass resin.

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