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Why you don't use bondo over metal

mightymidget

Jedi Knight
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today I started tinkering with Bugeye, seems the front of my bugeye was in a wreck and bonnet should have been replaced, but instead someone got real creative with bondo, check it out 1" thick, all of the rust is from the bondo. BONDO lets moisture pass through DO NOT use it on bare metal. Dynoglass is waterproof and will not get rust between it and the metal, but epoxy primer should be applied on bare metal!!!!



the headlight buckets have been welded in place and don't know if I can save this bonnet or not



I was using a hammer and wood chisel to pop the bondo out, I was really disapointed in what I found today

 
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Yow! I think you might have to write off that bonnet. I'm guessing that when you sand blast it, the metal is going to be VERY thin.
 

tony barnhill

Great Pumpkin - R.I.P
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I'm guessing all that rust was there before the bondo was laid on it - & I'm also guessing the bondo was laid directly over the rust without any bodywork or prep.
 

scoutll

Jedi Knight
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That looks overwhelming to say the least
 
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mightymidget

Jedi Knight
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No, this is a very old repair and I have seen this before. The metal is beat to death with a body pick, and then bondo packed on top metal, layer after layer of bondo and the heat builds up and moisture is under bondo from the get go. I am getting ready to post photos on the the rear of car where dynoglass was used and the only place rust started was were bondo cracked
 
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mightymidget

Jedi Knight
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I moved to the rear of car, notice the small cracks in bondo on drivers side rear, this car was hit on passenger side rear and dynoglass and bondo covers the complet rear of car


as you can see there is very little rust on this rear panel, the places where bondo did touch metal there were rust etchings


notice the only major rust area is right below where crack in bondo was. This will be a bondo reskin job, since rear panel replacement is out of the question.

 

blkcorvair

Jedi Knight
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My Midgets Tail was rear ended Pounded out and Bondo'd over. Looked quite the same. I dont blame the bondo, Just the shoddy prep work. When the metals not prepared correctly and the bondo is slapped over it, it can only last so long. Unfortunatally in yester year this was very common practice in fixing these old cars. Todays paint standards just show up many more flaws so prep work must be premium.
 

nomad

Yoda
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Seems like most of the problems usally show up where some clutz has layed it on an inch thick. My BE rear end was accessed for colission repair by cutting a hole under the licence plate so you can get your hand inside for hammer and dolly work. After the work a panel was bolted over the hole. Under the plate and not visible. Just a helpful tip!!
KA
 

Bayless

Yoda
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A little bondo may be one thing if prepped right but an inch is no way acceptable. I bought a VW Beetle for my daughter several years ago. It had a fresh 20 foot or so paint job. Soon turned into a 50 foot job as the primer continued to shrink. After a couple months, it looked the DPO had done his final prep with 36 grit paper. In taking all that off, we found not bondo but plain lacquer primer up to 1/2 inch thick in places on the roof.
 

blkcorvair

Jedi Knight
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Once bondo is applied as much as 1/8" I agree I think its effectiveness is dimmed. Firm believer that is great for skim repairs, but fot shaping panels that are otherwise unrecognizable. Well thats not what it was meant for. my 2 cents.
 

spritenut

Luke Skywalker
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Epoxy primer over bare metal before bondo.
Do NOT use etch primer over bare metal as bondo will not stick properly to the etching primer.
Etching primer is for aluminum if you want the top coat to stick better.
It works great on some rusty parts too.
 

tony barnhill

Great Pumpkin - R.I.P
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:iagree: Frank is correct....there are some of the newer materials that can go directly on bare metal, but the old stuff needs epoxy primer.

Actually, in my opinion, every car needs epoxy primer under the paint!
 

Bayless

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I'm pretty sure that epoxy primer is your best insurance against rust, as Tony says, under any paint. Be sure to check the instructions though. Most manufacturers specify that it must be topcoated within 24-72 hours. PPG recommends if longer than 72 hours to sand lightly with 400 and apply another light expoy coat then topcoat.
 

blkcorvair

Jedi Knight
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Bayless said:
I'm pretty sure that epoxy primer is your best insurance against rust, as Tony says, under any paint. Be sure to check the instructions though. Most manufacturers specify that it must be topcoated within 24-72 hours. PPG recommends if longer than 72 hours to sand lightly with 400 and apply another light expoy coat then topcoat.

Been there done this. Coated mine after most the body work was done to prevent flashing. (Tub was media blasted.) Than went back two weeks later sanded, Epoxy Primed again, and painted/ cleared all in a weekend.
 

erstearns

Jedi Trainee
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Anyone ever tried lead? Hard to work with I know but seems better if you want to fill deeply in areas hard to get to, as in rear of BE. Cutting hole under license plate seems like a good idea to get at rear quarter. Need to appy over bare metal and apply heat so makes sense to do before priming.
 

Bayless

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Lead absolutely has to be done on bare metal, before any priming. It is much trickier to use, particularly on a vertical surface. There is a pretty small temperature range requirement. Too cool and it won't melt and much beyond that and it just runs off. But lead is certainly much more permanent and any significant thickness. You have to be pretty good though to get that final finish since you can't sand it but only file instead. Unless you are a real purist, a very thin coat of bondo, or better yet, polyester glazing putty is usually acceptable.
 

chrpark

Senior Member
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I've been plowing through some of the DuPont technical stuff to sort out the plan for my '62 MkII after media blasting; seems that etch primer right after media blasting to prevent any flash rust from forming, followed by coat of epoxy primer-filler; then necessary panel repairs, straightening, thin filler coats etc, lots of sanding, then coat of primer/sealer followed by finish coats and clear. That's three kinds of primer but each with a specific purpose.
 
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mightymidget

Jedi Knight
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If you spray bare metal with epoxy primer can you top coat with sandable primer to cover until ready to prep for paint. I want to strip & prep car in sections. it may be a year till I paint complete car. that way when I am ready to block sand complete car I can just sanf off sandable primer
 

tony barnhill

Great Pumpkin - R.I.P
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Yes...we always use sandable primer after epoxy because epoxy shows all faults & we can do final repairs.
 

Bayless

Yoda
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I don't really know about DuPont but most epoxy primer, including PPG is not a filler. As Tony says, every little flaw just shows right through the epoxy. I suspect any epoxy would be too hard to sand for a filler anyway. The best method I have found is epoxy over the bare metal. Very thin bondo can go over the epoxy too but better if it is under. Follow that with a primer/surfacer but be sure it is catalyzed. Lacquer will shrink for a very long time and may not be very compatible with a modern topcoat anyway. After you get the surfacer finished like you want it then apply another coat of epoxy but reduced about 50% this time. That is an excellent sealer and the topcoat can go over it. Using epoxy this way, it is safe to do a section at a time, excluding the topcoat.
 
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