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How to fix bondo cracks?

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Old 03-27-2005, 05:45 PM   #1
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How to fix bondo cracks?

Hey guys a buddy of mine had some work done on his truck, like the handles shaved and tailgate handle. Its been months now and now that its getting warmer the bondo has started to crack....or maybe its the primer? Im not entirely sure, however the cracks are sprouting up everywhere . What would you guys suggest to do? Sand off all the bondo and redo the doors? If at the worst, cut out the section that it was shaved since the bondo is cracking i bet its just layer upon layer of bondo and weld a new section in and use less bondo? What would you guys suggest? thanks in advance, its much appreciated!
Old 03-27-2005, 05:51 PM   #2
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Re: How to fix bondo cracks?

id grind it down and see whats causing it, they may have the filler stacked to thick, or didnt sand it good enough before filling, or they might not have fully welded the filler plate, just gotta grind it and see how bad it is then go from there
Old 03-27-2005, 05:52 PM   #3
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Re: How to fix bondo cracks?

Quote: Originally Posted by 19sTEN96
Hey guys a buddy of mine had some work done on his truck, like the handles shaved and tailgate handle. Its been months now and now that its getting warmer the bondo has started to crack....or maybe its the primer? Im not entirely sure, however the cracks are sprouting up everywhere . What would you guys suggest to do? Sand off all the bondo and redo the doors? If at the worst, cut out the section that it was shaved since the bondo is cracking i bet its just layer upon layer of bondo and weld a new section in and use less bondo? What would you guys suggest? thanks in advance, its much appreciated!
If the bondo is craking than there is TOO MUCH on the truck, the bondo should have only been used to straighten imperfections, from welding and grinding. I would sand the doors down again and make sure that a filler plate was used, if not order up the filler plates , weld them in, grind the the tack welds, and fill as needed. , Never ever use more than an 1/8in of filler. and make sure thge surface is clean, and well prepped for the filler.
Hope this helps.
Nick
Old 03-27-2005, 08:20 PM   #4
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Re: How to fix bondo cracks?

Personally I think the 1/8 inch thick is a myth. Usually that's all that's required in actual body work, people are using body filler now in just big mounds and using small die grinders and sandpaper to sculpt out designs and faces and all kinds of stuff. So obviously that's gonna be more than an 1/8 inch thick. Personally, I couldn't, so if I want a skull molded in to something I'm gonna grab a skull mold I have and use fiberglass cloth and mold it in, but that's just me. If the filler is cracking where something was shaved, he's getting stress cracks. More than likely they didn't weld the filler in all the way and then just tried to use filler to fill in the gaps. It works fine on a door handle that doesn't have a lot of stress, 90% of the time. But there is the chance for stress cracks. If you were to do it on a rear full skin combo, it would crack within about 2 days because it's a high flex area. The shop should have welded to filler in completely or until there were atleast tack welds every 1/4 - 1/2 inch apart. But once the welds were ground down, the first layer should have been some body filler w/ fiberglass strands in it. Just on the weld seam. something like dynaglass or evercoat glass lite, something like that. It's commonly referred to as kitty hair. That gives your weld seam more strength and holds up well. Once your seam is sanded, then you can proceed to regular filler to smooth the area out. I've even seen some guys go to big extremes and take fusor and fiberglass cloth and layer that over where they shaved something. I may do that on a tailgate skin, but I never have on a door handle before. Thing about filler is, you can spread it on a little thick or heavy if you need to, because 80-90% of it, is going to get sanded off. Since you mentioned that he had his handles shaved, if I had to guess, there's nothing wrong with his actual filler, the root of his problem is more than likely stress cracks.
Old 03-28-2005, 05:09 PM   #5
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Re: How to fix bondo cracks?

Quote: Originally Posted by BigBassInside
Personally I think the 1/8 inch thick is a myth. Usually that's all that's required in actual body work, people are using body filler now in just big mounds and using small die grinders and sandpaper to sculpt out designs and faces and all kinds of stuff. So obviously that's gonna be more than an 1/8 inch thick. Personally, I couldn't, so if I want a skull molded in to something I'm gonna grab a skull mold I have and use fiberglass cloth and mold it in, but that's just me. If the filler is cracking where something was shaved, he's getting stress cracks. More than likely they didn't weld the filler in all the way and then just tried to use filler to fill in the gaps. It works fine on a door handle that doesn't have a lot of stress, 90% of the time. But there is the chance for stress cracks. If you were to do it on a rear full skin combo, it would crack within about 2 days because it's a high flex area. The shop should have welded to filler in completely or until there were atleast tack welds every 1/4 - 1/2 inch apart. But once the welds were ground down, the first layer should have been some body filler w/ fiberglass strands in it. Just on the weld seam. something like dynaglass or evercoat glass lite, something like that. It's commonly referred to as kitty hair. That gives your weld seam more strength and holds up well. Once your seam is sanded, then you can proceed to regular filler to smooth the area out. I've even seen some guys go to big extremes and take fusor and fiberglass cloth and layer that over where they shaved something. I may do that on a tailgate skin, but I never have on a door handle before. Thing about filler is, you can spread it on a little thick or heavy if you need to, because 80-90% of it, is going to get sanded off. Since you mentioned that he had his handles shaved, if I had to guess, there's nothing wrong with his actual filler, the root of his problem is more than likely stress cracks.

the only time I have evers seen filler crack is from a stress crack, How else is it gonna crack??
Anyway, just follow my advise and let us know what you find.
Nick
Old 03-28-2005, 05:17 PM   #6
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Re: How to fix bondo cracks?

Quote: Originally Posted by bodydropped93
If the bondo is craking than there is TOO MUCH on the truck, the bondo should have only been used to straighten imperfections, from welding and grinding. I would sand the doors down again and make sure that a filler plate was used, if not order up the filler plates , weld them in, grind the the tack welds, and fill as needed. , Never ever use more than an 1/8in of filler. and make sure thge surface is clean, and well prepped for the filler.
Hope this helps.
Nick
Yeah i agree. Bondo should only be used to make evreything smooth. And your supposed to sand most of it off. I would sand it down to the bare metal. make sure its clean, and theres no oils from your skin or anything on it, And just start over. Thin layer of bondo, sand it smooth, and primer it over. Because generally when it starts cracking, there isnt much you can do to "fix" it other than do it over again.
Old 03-28-2005, 08:09 PM   #7
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Re: How to fix bondo cracks?

Quote: Originally Posted by bodydropped93
the only time I have evers seen filler crack is from a stress crack, How else is it gonna crack??
Anyway, just follow my advise and let us know what you find.
Nick

Haven't used much filler there have you buddy. A stress crack is only at a weld seam. You could have filler start to crack at a place where you were just smoothing out a dent and it's not a stress crack, and it's caused from several different things. Improper mixing of the hardener, I've seen it crack from people not scuffing up the paint or grinding it down to the metal. It can crack from people trying to mix filler and putties together that weren't intended to be mixed together. Lot's of things. but like I said, if he has a crack where a handle was shaved, it's a stress crack and needs a layer of fiberglass strand filler over the weld seam, then bodywork. Evercoat Tigerhair is a good brand to use.
Old 03-28-2005, 08:18 PM   #8
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Re: How to fix bondo cracks?

ya just sand it back down and see where the problem is, my shit starting cracking too, im gonna sand it back down and use kitty hair and bondo this time
Old 03-28-2005, 08:25 PM   #9
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Re: How to fix bondo cracks?

at the shop i work at we always put fiberglass over welds because its less likley to show through over time and is 100 times stronger than bondo then we put a light skin oh bondo over that to smooth it out for primer
Old 03-29-2005, 07:00 PM   #10
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Re: How to fix bondo cracks?

Quote: Originally Posted by BigBassInside
Haven't used much filler there have you buddy. A stress crack is only at a weld seam. You could have filler start to crack at a place where you were just smoothing out a dent and it's not a stress crack, and it's caused from several different things. Improper mixing of the hardener, I've seen it crack from people not scuffing up the paint or grinding it down to the metal. It can crack from people trying to mix filler and putties together that weren't intended to be mixed together. Lot's of things. but like I said, if he has a crack where a handle was shaved, it's a stress crack and needs a layer of fiberglass strand filler over the weld seam, then bodywork. Evercoat Tigerhair is a good brand to use.

As a matter of fact, I think I have my fair share of body work experience, considering I have restored custom hot rods, and in the middle of finishing a 3/1/2 body drop on a first generation, with everything shaved, So I would defininatly say I know what I am doing.
Old 03-29-2005, 09:21 PM   #11
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Re: How to fix bondo cracks?

Quote: Originally Posted by bodydropped93
As a matter of fact, I think I have my fair share of body work experience, considering I have restored custom hot rods, and in the middle of finishing a 3/1/2 body drop on a first generation, with everything shaved, So I would defininatly say I know what I am doing.
Well considering body drops and restoration work is mainly welding anyway, you should know how to weld, I'm not disputing that. But as far as actual body work is concerned with fillers, there's a lot of other reasons it will crack, and stress cracks are at a weld seam, which is why you need to use the kitty hair. Evercoat Tiger hair like I said is fairly pricey, but it's well worth the money. All evercoat fillers and putties are good quality. The kitty hair has strength, and the filler is more for cosmetic purposes that anything else. and as long as you put it on lightly and scuff the existing layers before adding more layers, you can build up filler and it won't crack from that. But 19sten, just tell your buddy he's going to have to re do it. I also would wait a few weeks to a month or so before repainting just in case you have a crack it's easily repairable and doesn't ruin a paint job. If you use the kitty hair, most of the time you can paint as soon as your bodywork and priming is done, but sometimes it's the old better safe than sorry philosophy for waiting to re paint.
Old 04-01-2005, 10:35 PM   #12
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Re: How to fix bondo cracks?

I'd recommend Metal 2 Metal filler
Old 04-02-2005, 06:36 PM   #13
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Re: How to fix bondo cracks?

Quote: Originally Posted by BigBassInside
Well considering body drops and restoration work is mainly welding anyway, you should know how to weld, I'm not disputing that. But as far as actual body work is concerned with fillers, there's a lot of other reasons it will crack, and stress cracks are at a weld seam, which is why you need to use the kitty hair. Evercoat Tiger hair like I said is fairly pricey, but it's well worth the money. All evercoat fillers and putties are good quality. The kitty hair has strength, and the filler is more for cosmetic purposes that anything else. and as long as you put it on lightly and scuff the existing layers before adding more layers, you can build up filler and it won't crack from that. But 19sten, just tell your buddy he's going to have to re do it. I also would wait a few weeks to a month or so before repainting just in case you have a crack it's easily repairable and doesn't ruin a paint job. If you use the kitty hair, most of the time you can paint as soon as your bodywork and priming is done, but sometimes it's the old better safe than sorry philosophy for waiting to re paint.
This guy knows what he is talking about.
Don't put the new filler or kitty hair over an area where you used like a 36 grit grinder or sander though - use like 60 grit.
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