Bondo Questions:

[Captain D]

Hi guys,

I'm a newbie here, so please bear w/ me. As I am sanding the body of my Charger down as close to the metal as I possibly can, there are some scratches that sanding just can't get out. A friend of mine suggested simply applying bondo to those areas and sanding them down. After all, everything, including the lightly bondo areas, will all be treated with a rust sealer and primer-ed.

That seems better (and cheaper) than purchasing whole new fenders, especially if they're only small blemishes/scratches. No big deal. But, I don't know a lot about bondo in general. Can you guys help answer some general questions that I have:

1.) how durable is bondo? I mean, how long does it last?
2.) does it ever become brittle?
3.) if I have some on my car, do I ever have to worry about it becoming loose once it's professionally painted (due to bumps and jolts while regular driving)?
4.) did they use much bondo on these cars 40 years ago? (reason why I ask is because someone told me that all cars have bondo on them. At the factory, they apply a layer of bondo before it gets painted - just curious to ask if they used bondo back then and how much).

I realize that using as little bondo is best, but I was just curious to see if I can learn more about it. My friend, who owns a bunch of cars, is really into bondo. He restored a car and had it painted. He said that it'll last for a very long time. Sure the car looks great (dang near showroom), but I just thought that I would ask you guys. At this point, and based on my limited knowledge of bondo, I'm "assuming" that a lot bondo is okay if you decide to keep your car just for car shows and parades, ect. Is that right? How long will those bondo areas on that particular car actually last?

Thank you again for any replies & for your time,
Newbie Aaron

[Chatt69chgr]

Nothing wrong with bondo as long as it's not too thick.  Bondo's bad rap came from people using it to do major body repair, like filling in big rust holes, instead of doing the proper sheet metal work.  Unscrupulous used car lots sometimes used to stuff old newspapers into holes and then Bondo right over the top of the hole rust and all.  Of course, it doesn't last.  I fixed a place on my old 73 Ford pickup sometime around 1977 with Bondo and painted it myself with acrylic enamel in my driveway with a borrowed Sear compressor and it was holding just fine when I sold the truck in 1992.  You have to do the requisite metal repair and prepare the surface correctly.  Many years ago, car companies used to use lead particularly in the area where the roof meets the rear quarter.  And Bondo was't available until the late 50's or early 60's. 

[RogerDodger]

Nothing wrong with bondo as long as it's not too thick.  Bondo's bad rap came from people using it to do major body repair, like filling in big rust holes, instead of doing the proper sheet metal work.  Unscrupulous used car lots sometimes used to stuff old newspapers into holes and then Bondo right over the top of the hole rust and all.  Of course, it doesn't last.  I fixed a place on my old 73 Ford pickup sometime around 1977 with Bondo and painted it myself with acrylic enamel in my driveway with a borrowed Sear compressor and it was holding just fine when I sold the truck in 1992.  You have to do the requisite metal repair and prepare the surface correctly.  Many years ago, car companies used to use lead particularly in the area where the roof meets the rear quarter.  And Bondo was't available until the late 50's or early 60's. 

Good advice.

The problem with Bondo is when moisture is able to get behind it it will bubble. Any rust needs to be properly repaired by removing and welding in new sheet metal. Limit the amount of Bondo by properly preparing the area. Scratches from sanding normally don't require Bondo. Use a high build polyester primer and apply about 3 coats. Block sand it starting with 120 grit paper. Deeper scratched can be filled with spot putty.

[CB]

Hi Aaron

Below a picture on how people 'restored' both quarter panels on my last 69 Charger, with some rivets, aluminum sheet metal and gallons of bondo.
Not a pretty sight once you decide to fix er up the right way. Meaning replacing the rotted out metal by new metal.
My body guy removed inches thick layers of bondo on my Charger, you couldn't believe it.
I am not a big fan of using bondo unless it is applied with sense. Like Roger says, high build filler should be enough to get a descent prep before paint. And after a good body man used his skills with proper tools.

[Captain D]

Thank you guys for the responses!

I appreciate your time & guidance. It sounds as if bondo does have its positive uses/benefits if used in a non-excessive way. I can understand that once water/moisture gets behind the bondo that it'll bubble out. But, this may sound like a dumb question to ask...but how does water/moisture get behind the bondo?

What I mean is...if there is metal (say on the inner side of a fender or quarter panel, even if it's only thin sheet metal) there is some form of metal backing there. And, even if it is treated w/ Rustoleum or undercoater or both, then moisture can still creep through all of that rust and moisture preventatives and affect the bondo from underneath?

The reason why I ask is because on my Charger, it looks like the previous owner repaired a spot. You can see the metal from underneath. When he was done, he treated it with a rust sealer and followed up w/ the rustoleum primer + paint. On the exterior, he applied a layer of bondo in order to help smooth it out. If I were to take one more step and apply undercoater underneath, will that bondo that is on top/exterior eventually bubble-up and rust out later on down the road even though it has those rust preventatives underneath?

I suppose the best thing is to avoid taking it out on rainy days to avoid water splashing up onto that particular area, but even though the car is protected & enclosed, moisture is still in the air so that's still something to keep in mind I guess.

Thank you once again for taking the time to answer my question,
Kind regards,
Aaron

[Mike DC]

BONDO --

Overall, just keep in mind a few things:

1.  The car basically just needs an unbroken steel skin under the paint.  No pinholes.  No big rust holes.  No non-factory metal patches unless the seams are 100% welded into place around the whole perimeter of the patch.    The car's sheetmetal doesn't have to be 100% gorgeous work underneath the smoothing layers just to hold up over time, but it needs to be an unbreached perimeter.

2.  Bondo is very good when it's thin.  It's very bad when thick.  A decent bodyworker will not like having bondo more than maybe 1/8" thick anywhere on a car he cares about.  If you're still getting 1/4-3/8" thickness of bondo on a panel when the major excess is sanded away then your metalwork underneath the bondo just isn't good enough.

3.  Primer and bondo are similar substances in the big picture, just different degrees.  (These days some of the top-end bodyworking product companies are actually starting to blur the line between these two.  They're offering primers that are thicker & thicker, and at the same time they're starting to offer bondo/filler coatings that are thin enough to be sprayed on.)

[Captain D]

I apologize for the delayed follow-up response, I've been away from the computer. But thank you once more for taking the time to answer my questions!

I'm finding that this thread is VERY helpful indeed...You guys are extremely knowledgable! It's good to know that a thin layer of bondo is best rather than thicker layers. And, that the underneath doesn't have to be 100% perfect (as long as it is all welded, seam-sealed, & treated properly w/ no pinholes, ect.). I think I've seen what you mean about primers getting thicker & thicker. I recently had seen several selections of the filler primer that one can choose from.

Thank you again for your time & help!
Aaron

[BlueSS454]

"Bondo" is a very often misued term.  Bondo is a cheap brand of body filler sold at stores like Pep Boys, Autozone, etc.  The correct term is plastic filler and when used properly, it serves it's purpose very well.  If you plan on doing any type of filler work, go with a higher end filler such as Evercoat's Rage Gold or Rage Gold Extreme.  It spreads and sands much easier than that cheap garbage you get at Pep Boys.  FIlled areas should not be more than 1/8 to 1/4 of an inch thick.  There are 2 ways to apply filler, over bare steel or over a surface that has been treated with epoxy primer.  I choose the latter of the 2.

[69*F5*SE]

 

[375instroke]

Evercoat for me.  Scratches are usually primered and then sanded, not Bondoed.  If you have a small low spot or some irregularity, Evercoat's Polyester Glazing Putty is what I would use, not that 3M spot putty I see some people smear over an entire door.  Bondo bricks falling off cars has more to say about the person who put it there in the first place, not the quality of the product or thickness of it.  All the bricks I've seen had paint on the back side of them, and it was the paint that came off the car, not the Bondo off of the paint.  Also the rust thing from trying to fill a hole with it as to what caused the failure.  Mopar used a Bondo product on the roof seams of vinyl top cars and lead on painted roof cars.