Wednesday, October 2, 2013

How to repair a rust out with fiberglass and body filler

     A fiberglass rust-out repair is inexpensive and effective. I have been doing these for years and they hold up well. After two or three years some touch-up may be needed around the edges. The biggest advantage of fiberglass: it doesn't rust.

                                           The Job Basics
  •  Safety: Glasses, gloves, earplugs, dust mask
  •  Tools: Electric drill, drill-mounted wire brush, Drill mounted paint remover, 4” sanding disc attachment, putty knife. Hammer and chisel, scissors,
  •  Materials: fiberglass kit (cloth, fiberglass resin and hardener), Body filler kit (filler and hardener), a tube of spot putty, newsprint, marker, sandpaper from coarse to very fine grit,  double sided tape, automotive primer, Auto body paint to match vehicle.
  •  Materials cost: About $25- $40 dollars. Fiberglass kit: used 1/10th of $10 kit = $1. Body filler: Used 1/3 of $8 kit = $2.60. Computer matched paint from Car Quest: used 1/5th of $20 can = $4 Primer used 1/10th of $3 can= $.30
  •  Shop cost: Does not apply. This job will add to the appearance and value of the vehicle when sold.
  •  Time:  estimated at 3-4 hours over 2-3 days.

                                                    Rough Surface Preparation

1) Start with the putty knife and remove as much loose paint and chunks of rust as possible. Look for bulged out paint. There is rust under there. Use a chisel and hammer to remove more loose material if necessary.

That's getting ugly. Time to fix it. 

2) Attach a rotary wire brush to the drill to remove more loose paint and rust. A special paint removing drill attachment helps remove the looser  paint.
3) Expose a  inch or two of bare metal from the edge of the rust-out by sanding it off with the drill-mounted round 5 inch sanding disk and coarse sandpaper.  Parts of the damaged area that are in difficult to reach will need to be removed by hand or with a small rotary tool such as a Dremel tool. I like to use the cut-off wheel.

                                            Cover the rust out with fiberglass cloth

1) Make a pattern of the rust out to transfer to the sheet of fiberglass. Tape a sheet of paper (newsprint works well) over the rust out and mark one inch beyond the edge of the rust out with a Sharpie marker. Cut it out and then put it back over the rustout with masking tape and make any additions or cuts as needed.
Now lay the paper pattern over the fiberglass cloth and trace it with the marker. Cut out the mat with a good scissors. Tape double-sided every few inches along the entire upper edge of the rust out. Now carefully apply the top of the fiberglass cloth to the line of tape and let it hang there.


                                                  Apply resin to the fiberglass cloth
1) Check the product label for mixing instructions. My Bondo brand fiberglass kit called for 1oz (28g) of resin to 14 drops of hardener. I only need a small batch for the initial tack so I mixed 14 grams and 7 drops.
2) Try to fit the fiberglass cloth follow the contours of the panel by whatever means possible. Strips of poster board or discarded cereal boxes taped to the panel to follow the contours work well. On this repair I needed to fit the cloth all the way under the panel, all the way to the part underneath the panel.  I applied another 16g of resin and 8 drops of hardener to fill in the upper right corner of the repair area and the lower left as well. I did these areas first because there was some contour to follow there. The middle, with so much material and contour missing will then follow those areas when they are hardened. Use some odd pieces of wood to hold the contour by bracing them from underneath. Keep adding resin until the entire cloth is filled in. Total fiberglass portion: about 1 hour 15 minutes.
                                                               Fill in with body filler
1) Next we will fill in the surface irregularities with body filler. Follow the can instructions. Start filling in the deepest depressions first with a thin layer each time. After 20 minutes the next layer can be applied. Probably a good dozen layered applications of filler later I am done.

2)  Sand it as smooth as possible, following the contours of the panel.
3) Fill in the surface imperfections with spot putty.


Now do a final sanding on the filler with a 100 grit or finer sandpaper.

                                       Prime and paint

1) Using masking tape and newsprint paper, screen off from overspray.
2) Wipe the area to be painted with lacquer thinner on a clean cloth.
3) Spray on the primer at about 10 inches from surface. Keep the can moving.

4) Sand the primer with a fine grit sandpaper and fill in any surface imperfections with more spot .putty and sand smooth again.
5) Repeat until satisfied that the surface is smooth and ready for painting.

Apply the paint from about 10 inches away in short bursts and keep the can moving. Put on a light layer and wait a few minutes. Apply another light layer and finally a third until satisfied with the finish.

The finished repair



  1. I was getting ready to repaint my car, and I found a big spot of rust on the body. Your step by step instructions as well as the pictures are very helpful to anyone that might be new to this as I am. I thought it would be a lot harder than it actually was to remove the rust from my car. Thanks for the great tips and tricks.

  2. It is a great website.. The Design looks very good.. Keep working like that!.

  3. I'm a high school senior planning on using this method to fix the rust at the bottom of my car . Fingers cross it goes well !

    1. Thanks for using this post and I wish you well on your work. Take your time and it should be fine.

  4. This comment has been removed by the author.

  5. Could you explain more about the step using cardboard to maintain contour?
    I'm getting ready to tackle a typical case of Nissan quarter panel rot and I'm not sure how to get the cardboard out once I finish the repair -- wouldn't that trap more water and moisture, or am I missing something obvious?

    1. The cardboard is just used on the initial fiberglass cloth application and stays on only until the fiberglass resin sets up. Any that might stick on there is removed during the sanding and body filling stage. Good question and thanks for using this content!

    2. The cardboard is just used on the initial fiberglass cloth application and stays on only until the fiberglass resin sets up. Any that might stick on there is removed during the sanding and body filling stage. Good question and thanks for using this content!

    3. OH! The cardboard wraps around the OUTSIDE.
      I thought I was supposed to stick it on the inside of the hole and form the fiberglas around it. I think I understand now.

  6. Very nice write up, one of the best on this subject I found so far on the Web. I'm wondering if I can spray the bare metal with primer before the first layer of fiberglass mesh to ensure the metal is sealed from moisture, or if it might inhibit the resin from bonding to the metal. Thanks!

    1. I just go by the instructions on the fiberglass resin product and they don't suggest putting primer over the bare metal, at least not the one I use. I like your thought process,though,for looking for an edge on beating the rust.

  7. Mold the fiberglass patch by making it on the *inside* of a paste waxed rust repair panel. The hardened fiberglass will pull out pre-shaped and smooth as glass. Wipe the molded part with solvent to remove both the wax from the mold and the wax the resin surfaces (its to slow the catalyst hardener from evaporating before it hardens.) Trim the fiberglass patch to fit your hole and use more polyester resin (and glass if necessary) to glue the molded fiberglass in place. The only shaping required is to blend the edges of the patch to the car.
    You already had a rust repair panel visible on the rocker panel. As you know they’re very inexpensive. They’re meant to be use by cutting out the area and welded in place of the metal removed. You can use the formed sheet metal as a low cost fiberglass mold.

  8. Those car rusts are looking really mess. Though mine one's looks more horrible. But with this free repair manuals for car is lot easier than it was before. It is absolutely free and available online website named

  9. takes 1 hour 15 minutes to do.

    Will last 1 hour 15 minutes until it rots out again.

    Fix it properly or sell it to someone who will, this shit is stupid.

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