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MoabJeeper Magazine Article

Technical / Installation Articles - Body Interior / Exterior

Bondo-Glass Fiberglass Repair

Article written by Moab Man

Date Added: 09/19/2008

Fiberglass is an amazing material. It's incredibly lightweight, strong, and can be manipulated into just about any shape.

In spite of all these great attributes, fiberglass has its limits: rocks, water, and ice. Rocks can bust it open, water sneaks inside, and the expanding ice pops the layers of fiberglass apart.

One of our hardtops took some damage a few years ago when we laid the attached TJ on its side. We didn?t think much of it at the time, but after a few years of Utah winters (thawing and freezing over and over again), ice had compromised the integrity of this Jeep hardtop; and it needed some immediate attention. Having seen plenty of other damaged hardtops on the trail, we thought we would show you how to repair the fiberglass of your hardtop.



1. Here are the supplies you will need to purchase: 1 quart can of Bondo-Glass which includes a tube of hardener, Bondo Glazing Spot Putty, Bondo Plastic Spreaders, sandpaper from course to fine, and a small spatula for mixing and applying the Bondo-Glass.

2. Here are the tools we used: Vibrating palm sander, sanding block, corner sander, Dremel Rotary Tool with grinding bit, face mask, air compressor (or vacuum), damp cloth, and latex gloves.

3. This is what we had to work with. Lots of cracking, layers of fiberglass no longer bonded together, and lots of loose fiber.

4. To start we need to get rid of the surface paint and loose particles, and examine what's hidden below the surface. We did this with our palm sander and some course sandpaper.
 WEAR YOUR FACE MASK! You do not want to get these glass particles into your lungs.
 You can see in the pictures were working outside. We wanted the light breeze of working outside to avoid a fiberglass dust cloud. This worked really well, but you want to keep your working area in the shade.

5. The course sandpaper made pretty quick work of the surface paint exposing additional damage not previously visible.

6. Due to the compression damage from the rocks, not all of the surface could be removed with our palm sander. For this we used our vibrating detail corner sander.

7. Here is the corner of our shell all cleaned up exposing quite a few underlying cracks. Once you have uncovered all of the cracking, you must extend your sanding out 1 - 2 inches further. This gives the Bondo-Glass a solid base material to hold the repair all together.

8. Following the cracks in the fiberglass, we could see they extended around and into the rear window channel. Wanting to do the job right, we removed the window seal to see the extent of the damage below. This seal slides straight back and off.

9. Now that the seal is out of the way, you can see the crack ran all the way to the end of the window channel lip.

10. Using our Dremel Rotary Tool, like a dentist going after a cavity, we ground away all the damaged, cracked material.

11. This next part?. just sucks. We ground away the damaged areas, but as we said earlier, you need to prep the surface further out than the damaged area for the repair. We really couldn't sand the area we wanted with the Dremel very well, so we had to resort to hand sanding. From experience, that's a pretty thick layer of paint on the fiberglass so it takes a while.

12. All the loose fragments have been removed, we've sanded all of the damaged areas with course sandpaper, used our compressor to blow off all the dust, and wiped the working area down with a damp cloth. Time to move on to the Bondo-Glass.
 If you don't have a compressor, a vacuum with a hose will work.

13. The instructions for using the Bondo-Glass are pretty straight forward.

14. We?d like to draw particular attention to the working time. The package says you have about 5 minutes working time. We recommend you get a big, easily visible clock to keep an idea on how fast time is flying. Once we got the Bondo-Glass mixed well and applied to the working area, we found we only had about three and a half minutes of working time before it was gelling.

15. Put a 1/4 can of Bondo-Glass into the included plastic cap/bowl.

16. Start your timer. You have five minutes total to get the next six steps done.

17. Add the proportional 1/4 of the tube worth of hardener. You will have to guestimate on the 1/4 tube since there really is no easy way of measuring it. This must be why additional Bondo Hardener is so cheap.
 If we had been smarter, we should have spread the hardener around in the bowl rather than creating a lump. It would have been easier and quicker to mix thoroughly increasing our working time. Fortunately, we didn't need additional work time in this instance.

18. Mix thoroughly.

19. When the Hardener and Bondo-Glass are mixed really well, you will see the color change from light green to a really terrible dark green

20. Apply liberal amounts of mixed Bondo-Glass to the working area using the mixing spatula.

21. Using the Bondo-Glass Spatula, quickly spread and press the Bondo-Glass into the cracks of the damaged area.

22. Cover the entire sanded area right up to the paint. A little overlap onto the painted area won't hurt.
 The instructions called for a 12-15 minute curing time. We gave it 45 minutes since some of the repair area was pretty thick and we didn't want to risk opening up a pocket not quite finished curing.

23. Having given the Bondo-Glass plenty of time to cure, it's back to the sander. This is where you will gain an appreciation for those that do body work and turn out amazing pieces of work.

Clean the dust from your working area, and wipe it down with a damp cloth before moving on.

24. Once you finish sanding/shaping the Bondo-Glass repair, there will be small pits or imperfections. Unless you have major fixes requiring another application of Bondo-Glass, Bondo Glazing Spot Putty will make quick work of small imperfections.

25. Squeeze a small amount of Bondo Glazing Spot Putty next to the imperfections needing filled.

26. Using a clean spatula, smooth the Spot Putty into the pits.

27. As you can see, we pretty much covered the whole area with Spot Putty... one little imperfection led us to the next. Using fine sand paper, sand the putty back down to the fiberglass leaving the putty in the imperfections. Again, clean up your working surface before moving on.
 Depending on how much work you want to put into this repair, you may have to repeat the Spot Putty a few times.

28. Satisfied with the way our repair turned out, we applied a filler primer to both fill any really small imperfections, and to prep the surface for painting.
 As you can see by the lack of light (and the time on the clock) it's getting late. Living here in the desert with virtually no humidity, we can work into the late hours without moisture affecting the working surface. If you live in an area with humidity and things start to get damp, stop for the night. Cover your work area and pick it up again in the morning.

29. Primer can be applied, sanded, and applied again until you get the surface finish you desire. Allow time in between sanding/applications for proper drying. Clean and wipe down with a damp cloth after every sanding or the next application will not stick.

30. Last coat of primer.

31. Having given all repair materials enough time to thoroughly dry, we applied paint to our primed surface.

32. Unfortunately, there is no way to match the faded paint of our hardtop. That meant we had to purchase plenty of paint, scrub the whole top down, and then paint the entire top. The worst part? Masking off the glass and hinges.

33. The final product.

Installation Issues

We noted that you should have latex gloves. Please wear them. I didn't wear them and forgot about needing to get the fiberglass into the rear window channel where the spatula would not fit. Due to the short working time of the Bondo-Glass, I used my bare fingers. As a result of not listening to that little voice in my head that kept telling me to put my gloves on (hate it when that voice is right), I could feel the glass fibers for the next three days.

Final Thoughts

If you have never worked with fiberglass (fiberglass mud) it's really not that hard. And for the repair we did, this was the best way to do it. If we had chunks of hardtop missing, we would have approached this with fiberglass matting and resin.

The actual repair was done in about 4 hours. Repair, paint, and all the drying time can be finished in one day if you start in the morning. We got a late start, so it took us late into the night.

If you do attempt a fiberglass repair, about the only things you can really mess up is to rush it (not allowing proper drying/curing) or fail to properly clean your working area. Dust or dirt will prevent adhesion of the materials. Take your time and be patient.

A special thanks is owed to my daughter, Little Dawg. She helped with all the pictures where time was critical. The doughnut in the picture was her reward the next morning (she looks rough after being freshly dragged out of bed) for helping out. And the doughnut... yes that is a dinner plate in the picture.


Purchase / Vendor Info

Vendor Name: Purchased locally.

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