How to repair fiber glass cracks in a car

Fiberglass is used in many different applications, such as automobiles, pipes, tanks, aircraft, bathtubs, swimming pools, and many more. Many times, fiberglass is used instead of metal, and is not corrosive and much lighter. Fiberglass is also softer than metal, which makes it easier to grind and repair when necessary.

When the fiberglass is cracked or fractured, it can be easily repaired by filling glass fiber. Filler is thick as putty and requires a hardening catalyst. Fiberglass padding for body repairs can be found at your local auto parts store or at your local marine food store.


  • Put on a respirator, and grind directly into the crack using a dye mill. Crush all the damaged fiberglass, forming a slot where the crack was. Slowly decrease the walls of the slot until they are almost flat.
  • Clean the surface of the repair with a rag and acetone. Let the acetone dry.
  • Mix the filler and catalyst together in a small bucket, following the packaging recommendations. The thick load is agitated with a stirring rod to mix thoroughly in the catalyst. After the filling is mixed it will take around 20 to 30 minutes before it hardens.
  • Apply the filler to the crack, and smooth it with a spatula. The filling must completely fill the crack, and be level with the surrounding fiberglass. Let the filling harden.
  • The soft sand filler, with 200 sandpaper in a palm sander. Sand filling until it mixes with the surrounding fiberglass, and has no visible edges.
  • Clean the repair with a clean cloth and acetone, and apply your primer and paint.