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Old 04-01-2012, 07:14 PM
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Primer before bondo ??????

I am rebuilding a 67 Camaro. the paint(20 years old)that was on it had little bumps (1/8 inch) every where even under the trunk lid. I assumed rust under the paint. what i found was. the last time it was painted it had been taken to the bare metal. There was some bondo then a very heavy coat of white ish (Primer) ?? on top of that was a thin coat of a brown primer, then the paint. The paint did not seem to stick to the brown primer that well, i could flick it off with a finger nail in a lot of spots. When the paint came off the brown primer seemed very smooth. The white primer stuck to the metal very well and the bondo seemed to stick well to the metal.
part of my question is that when I sanded every thing off as i went through the bondo there was a fine layer of surface rust on the metal.
SO to my questions!!!
I am using epoxy primer. DO I WANT TO PRIME BEFORE I BONDO????? will the bondo stick to the primer? will the primer stop the surface rust that i had? dose it matter?
I am not going for the 100 point show car. I am going for a bad *** driver that looks good up close.

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Old 04-01-2012, 08:58 PM
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I'd sand it down to clean metal with some 80 grit on a D/A, then epoxy it and do the bondo work over that. As long as you put the bondo on the epoxy while it's in the recoat window it should stick just fine, though you could sand with 220 too if you really wanted. When that's done I'd spray another coat of epoxy over the bondo spots and then go on to the build primer. That way the metal is sealed from the bondo (epoxy is air and water tight once it flashes), keeping the rust you're seeing from forming again.

Basicly The answer is yes to all your questions.

Last edited by MikeyBob; 04-01-2012 at 09:06 PM.
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Old 04-01-2012, 09:12 PM
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I am old school, and use the system I grew up on. It was standard proceedure for about a hundred years... but now I hear that it isn't any good. I have put my bondo on bare metal for 40 years with no rusting.

I do sand the metal immediately before the plastic goes on. If the bare metal has set for a while... I metalprep it, then resand before bondo. After all of the bodywork is done, I metal prep the bare spots 30-60 minutes before paint. Metalprep is about $20 a can.

I'm sure you can do it either way satisfactorily, but I will warn of these experiences....

I recently completely stripped... bodyworked... and repainted an AMX. This had already been done. They relyed on the epoxy to prevent rusting, but the entire car had rusting under the epoxy, so it ALL had to come off... paint AND bodywork... and be completely redone!

I also recently painted a street rod that came to me with the body roughed out, and in epoxy and then 2K primer. They did not sand the epoxy before the 2K... and relyed on the time window for adhesion. Now the 2K is pulling away from the epoxy in the corners, and doing what is sometimes called "bridging". Sand the epoxy!!! It's easier than trying to repair the paint later!
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Old 04-02-2012, 06:54 AM
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I run into surface rust under paint all the time on the old stuff...I use Ospho to remove the rust and prep the steel before I epoxy..One of the reasons I like to epoxy first is it protects the surface from all kinds of contaminates,even the oils and salts in your hands are enough to give you trouble...I always sand my epoxy before any filler goes on but with 180 ,its rough enough for the filler to stick to and fine enough so that it dosent take all my epoxy off....after the filler work is done a couple more coats of epoxy to seal it all up....
I use the filler first method on smaller repair jobs and dont use any epoxy, just a 2K.
Basicly if you strip to metal you really want to use an epoxy...and dont think all epoxies are the same,look into what brands the pros use and why ,the guy at the paint store just wants to sell you something they carry...
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Old 04-02-2012, 08:47 AM
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I agree with deadbodyman, and here's why.

Filler is a porous material and when you apply it directly to freshly sanded bare metal and it isn't sealed quickly (few hours to a few days depending on the humidity where you live) moisture can infiltrate the filler and start rusting the metal underneath. This doesn't always happen but it can. Sealing the surface with a high quality epoxy primer first and then scuffing it is cheap insurance against trapping moisture in the filler which means your repair will last a lot longer.

I'm sure TucsonJay's method works very will where he lives as the normal humidity level is very low in the deserts of Arizona, and I doubt he leaves filler on very long before priming, but for the average backyard mechanic who may take a few weeks to do bodywork on his project, and in your case given the humidity in Washington state, I would definitely use epoxy under the filler.

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Old 04-02-2012, 12:06 PM
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All I know is for the past 35 yrs I've been putting filler over bare
metal and never had a problem. It'll outlast the car.
I've seen filler over 50 yrs old, too thick, and still holding fine.
If you think primer first is better, fine, but it's a moot point.
Either way it'll last longer than the car if preped right.
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Old 04-02-2012, 02:34 PM
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Quote:
Originally Posted by jcclark
All I know is for the past 35 yrs I've been putting filler over bare
metal and never had a problem. It'll outlast the car.
I've seen filler over 50 yrs old, too thick, and still holding fine.
If you think primer first is better, fine, but it's a moot point.
Either way it'll last longer than the car if preped right.
+1 Never seen it put on any other way.
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Old 04-02-2012, 02:48 PM
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If the OP wants to do a search he'll find this subject has been discussed at length many, many times with no real resolution. There are the epoxy first guys and the bare metal guys, and there's very little chance of convincing one side or the other to change their opinion.

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Old 04-02-2012, 05:35 PM
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For years we were told bare metal only. Now its epoxy first, oem is saying scuff and put it on e-coat. All work fine its just what do you feel comfortable with. I do both epoxy on some bare metal on others. If you have the whole car to bear metal epoxy first because for sure there will be surface rust before you get whole car ready for paint and I dont care how fast you are.
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Old 04-02-2012, 07:31 PM
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Ive been using Epoxy over bare metal before Bondo but.................Ive been reading so much about what it takes to fully cure Epoxy that its beginning to scare me. Epoxy seems to have a smooth finish, would that keep bondo from adhering?
I did Bondo over Epoxy and was blowing a small hole with my blow gun to blow all dust off.. about in inch from where I was blowing I heard a 'Pop!' ..The paint lifter and it separated between the Epoxy and the thin layer of bondo. What could have been the cause? as I took of a pretty bich chunk I played with he layers Epoxy filler, Bondo, High built Featherfill, High builf Urethane Primer... was was able to easily peel between the epoxy layer and the Bondo Layer pretty easily
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Old 04-02-2012, 07:39 PM
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Epoxy first for me.
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Old 04-02-2012, 07:42 PM
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Quote:
Originally Posted by 67Elcamino
Ive been using Epoxy over bare metal before Bondo but.................Ive been reading so much about what it takes to fully cure Epoxy that its beginning to scare me. Epoxy seems to have a smooth finish, would that keep bondo from adhering?
I did Bondo over Epoxy and was blowing a small hole with my blow gun to blow all dust off.. about in inch from where I was blowing I heard a 'Pop!' ..The paint lifter and it separated between the Epoxy and the thin layer of bondo. What could have been the cause? as I took of a pretty bich chunk I played with he layers Epoxy filler, Bondo, High built Featherfill, High builf Urethane Primer... was was able to easily peel between the epoxy layer and the Bondo Layer pretty easily
You need to scuff the epoxy because the filler needs a mechanical bond. The main reason for using epoxy in the first place is to protect and seal the bare metal from moisture, not to promote the adhesion of the filler. So, always scuff the epoxy to provide a good mechanical bond for the filler.

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