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Fiberglass repair for small hole in front fender

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Old 05-21-2005, 11:58 AM   #1
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Default Fiberglass repair for small hole in front fender

Hi Gang,

Looking for tips to fill the hole that was left when I removed the security alarm from my 68 convertible. The hole is on the front fender about 1 inch above the second vent behind the front wheel. It is about 7/8 of an inch in diameter. It was where the key mechanism to disarm system was mounted.

I bought a Bondo repair kit for fiberglass that includes fiberglass resin, hardner and fiberglass cloth. My plan is to sand the interior of the fender wall around the hole build several layers of the glass around the hole from the inside. When that is hard I figure I can then use some type of fiberglass filler on the exterior of the hole. Smear on several layers then sand and paint.

A few questions.

1) It looks like I can only use the fiberglass resin and hardner supplied with the kit in conjunction with the cloth. Therefore, I think I need a filler for fiberglass like SMC for the exterior? Any suggestions.

2) Also, I have seen fiberglass joint tape used and then body filler smeard over top of this on the inside of the repair. This is the technique used in my "Corvette Projects Book". And supposedly this "adhesive backed fiberglass mesh drywall joint tape" is available at home depot. Anyone ever use this?

3) Should I taper the hole from the outside so the filler fits on like a flat head wood screw? I am thinkng this will help with a smoother fill and less likey to crack down the road.

Many thanks in advance.

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Old 05-21-2005, 12:34 PM   #2
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The problem you will encounter is gravity. The fiberglass resin flows easily and will not stay put in the hole until it begins to cure. The cloth should be used on the back only...follow the directions and soak the cloth in the resin&hardener mix...the mix will tend to stay in the cloth better, but you will want to keep spreading the mix onto the cloth and keep it from drooping down...leave it be when it begins to get sticky, cause it dries very quickly.

Next will be to fill the hole...the fiberglass resin is great for that as well...just keep it from drooping and apply in stages...about three applications is all you will need. I had to add about 1/2" to my new front bumper to make it match my fender and it came out great. I highly recommend that you get some masking tape...the blue 4" wide stuff...and some 4 mil plastic to mask around your hole so that you don't mess up the rest of the body.

Sand with a vibration sander using medium grit like a 240 to 400 to get the contour correct, then move to a 400 and get the surface smooth. Get some of the (can't remember exactly what it's called) paint that is designed to help you spot imperfections...when you get it smooth then hand sand with 1000 grit and wash well with detergent to get all the dust off of it. You will of course need to prime and paint, but the technique of blending (even the primer needs this technique) is something that only experienced or very lucky beginners can do, so I recommend taking it to a paint & body shop...preferably one that handles classic cars if you want the best finish.

Good luck!
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Old 05-21-2005, 02:07 PM   #3
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I would use fiberglass mat instead of cloth. You might be able to get by if you only use it on the back of the hole though.
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Old 05-21-2005, 04:35 PM   #4
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The mat (randomly stranded) is much better than the cloth (woven even strands). The cloth may show up as a pattern in your paint over time.
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Old 05-21-2005, 06:40 PM   #5
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1. sand behind the fender to the fiberglass
2. tamper the edges of the circle..."v" groove it so it tapers.... the more of a "v" the better .... feather to flat
3. clean off the dust and use mat with resin to back it... just enough resin to soak into the mat.....if u have too much just squeeze it out with figer tips
4.position it on the back of the hole and let it setup....once solid, begin layering small strips over the outer side of the fender..mat and resin... lay 2 layers and let it setup
5. make sure there are no air pockets/bubbles
6. everyso often, sand the new resin up a bit to help get rid of air pockets and create a better surface for the next layer to grip to.
7. once u have built it up enough to where u are satisfied...possible 5 layers.... sand to the countour with a long board/block
8. use duraglass to reshape if need to be
9. skim coat it with a glazing coat and resand


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Old 05-21-2005, 06:59 PM   #6
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Yep, Good advice...

Also never use fiberglass resin without the glass filler. It will be very weak and brittle.

For filler, to expand of above message, use a short strand fiberglass filler to fill in the big spots.
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Old 05-22-2005, 12:52 AM   #7
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taper the outside of the hole
fiberglass the back of the hole- about 2 layers
then cut circles out of the mat that will fit inside the hole.
fiberglass those in so you won't use so much filler
progressively make the circles bigger so they follow the taper
the last piece will be almost level with the body
use regular bondo to smothen it out because it shouldn't need that much anyway.
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