How to fix spider cracks in fiberglass

Spider cracks are a form of fiberglass damage that occurs from bending or minor impact. They can occur in any form of fiberglass, including cars, boats and swimming pools. Small cracks are formed on the outer surface of the fiberglass, extending from a central point, like a spider’s web. These cracks do not affect the structural integrity of the fiberglass, but they are an aesthetic problem. Fortunately, spider cracks are not difficult to repair.


  • Clean the area with acetone and a rag to remove any dirt or other impurities.
  • Sand the damaged area with 100 grit sandpaper to scrape the surface. You can sand by hand or with an electric sander.
  • Wash the area with acetone a second time to remove sanding dust.
  • Mask the area around the cracks with duct tape and plaster to protect against drips.
  • Pull all the loose bristles out of a cheap brush. Loose bristles can be embedded in the gel coat.
  • Measure a small amount of gel coat, which is a thick form of fiberglass resin, into a paper cube. If the fiberglass is exposed to water, the gel layer must be marine grade.
  • Mix catalyst in the gel coat following the instructions on the package. Accuracy is important: if too much catalyst is added the gel coat will heal faster than it can be used, but if too little is added it will not heal at all.
  • Brush a thin layer of gel coat over the sanded area, fading out a little on the surrounding fiberglass surface.
  • Allow the gel-coat to sit for three hours, then lightly spray with polyvinyl alcohol to help it heal completely.
  • Allow the gel coat to cure for 24 hours.
  • Wash the Polyvinyl alcohol with soapy water.
  • Sand the surface with 300-grit sandpaper to begin smoothing. Re-sand with 600 grain, then 900 grain and so on. Keep jumping 300 grit at a time until repeated sanding has produced as smooth of a finish as you require.
  • Clean sanding dust with acetone.
  • First and paint the fiberglass to match the surrounding surface, if necessary.

Tips and warnings:

  • Wear rubber gloves when working with gel coat.
  • Work in a well-ventilated place.