How to use fiberglass to fill holes in a car

Updated February 21, 2017

As automobile owners drive their cars longer between trade-ins, wear and tear becomes a more frequent problem. When cars age, rust becomes a problem, most often making its appearance as a relatively small hole (rust out) in a fender or rocker panel. If addressed early, small rust holes can be repaired to prevent further spreading of the problem.

Sand the rust out and the area around it using a number 40 (coarse) sandpaper disc on a electric sander. Sand until no rust is seen beneath the paint and sand until bare metal only is exposed (no rust).

Cut away edges of metal if rust surfaces remain. Expand the hole until all rusted edges are removed.

Clean the sanded area using naval jelly applied with a rag. Naval jelly will neutralise any remaining and unseen rust. Allow naval jelly to work for 30 minutes and then wipe surface clean.

Insert heavy screwdriver into the hole and wedge downward around the perimeter of the hole to recess the metal edges. The objective here is to create an indentation around the perimeter of the hole that will provide a pocket for the fibreglass patch. If necessary to create this indentation, use the ball peen head of a hammer to indent the edges, creating a pocket 1/2 inch in width around the perimeter.

Cut fibreglass patch in the shape of, but slightly larger than, the rust hole. The patch should be large enough to cover the hole but small enough to fit within the depression made around the perimeter of the hole in Step 4. Insert the patch into the recessed area, being sure that the adhesive side is down and makes contact with the metal edges.

Mix body filler and hardener according to directions on body filler. Using putty knife, apply mixed body filler in thin layers, beginning with filling the hole, gently covering the glass patch and then the 1/2-inch-wide depression ring surrounding the hole, and continuing up to 2 inches beyond the perimeter of the rust-out hole.

Allow body filler to cure overnight before sanding and feathering edges with number 80 sandpaper.

Soak one piece of number 240 sandpaper in water and use to further sand the patched area 1/2 to 1 inch into the surrounding paint by feathering the edges. Keep sandpaper and sanded area wet.

Cut a hole in several pages of newspaper 2 inches larger in diameter than the patch and position this hole over the patch, taping the newspaper in place with masking tape on four sides. This will serve as a paint shield during spray painting.

Use masking tape to cover any chrome or painted surface in the vicinity of the area to be painted.

Shake the spray can of primer well and hold 14 to 16 inches above the surface while spraying. Apply a very light coat to prevent paint from running, using horizontal hand motions while spraying

Allow primer coat to dry 30 to 40 minutes before sanding with number 400 sandpaper that has been soaked in water. Sand until primer surface is smooth.

Clean primed area with rubbing alcohol applied with a clean rag.

Allow surface to dry completely (3 to 4 hours) and apply final paint colour following the same procedure as for applying the primer coat. For final colour painting, apply multiple, thin coats with drying time between coats to avoid running and blotches.

Buff to smooth finish, after paint has dried for 3 days, using a commercial polishing compound.


Fibreglass kits sold in auto parts stores vary slightly in application. Some have an adhesive side; others do not. Follow instructions coming with patch kit. When using body filler (Bondo), it is critical to follow exactly the mixing proportions for mixing body filler and body filler hardener. Incorrect mixing of the two can result in a defective patch, which might crack and fail.


Wear rubber gloves when using naval jelly. Do not attempt to apply body filler during rainy days, as high humidity can affect bonding.

Things You'll Need

  • Electric drill with a sanding disc attachment
  • Sanding disc, coarse grade (number 40)
  • Sandpaper (numbers 80, 240 and 400, wet or dry)
  • Naval jelly
  • Body filler (Bondo)
  • Sheet of auto fibreglass patching fabric
  • Spray primer
  • Rubbing alcohol
  • Masking tape
  • Auto spray paint
  • Polishing compound
  • Heavy screwdriver
  • Metal shears
  • Newspaper
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About the Author

Josh Weber is a retired industrial engineer. He has called on his engineering experience to write how-to articles for Associated Content, Demand Stuios and a business publication, "The Oyster Pointer." He is a graduate of The Virginia Military Institute and has a B.A. in economics and history.