Why Can’t Body Filler Be Applied Over Self Etch Primer?

(rock music) – Hey, Donnie Smith here, and I’ve just got a quick Q and A. I have a question on my Youtube channel, my DIY Auto body and Paint channel, and I was going to answer this question. It’s a good question, comes from Simon. It says, “Hey there, I
just can’t understand “why it’s not possible to apply
body filler to etch primer? “What happens when you do? “Is it just a question of bad adhesion “or does it react somehow
with the etch primer? “Cuz to me, it sounds even
worse to apply body filler “direct to metal since
it is porous material “and simply the moisture
in the air would get “trapped under the filler as well. “In my mind, you need to apply
something that chemically “merges with the metal,
like acid helps etch primer, “or something that transports any moisture “to the surface or otherwise
away from the metal. “Good vids by the way, “this is not a critique, just wondering.” And that is a good question. And the short answer to this is, you know, if you’re wanting
to apply body filler over a primer, like
you’re mentioning here, about the only primer that is recommended on the technical data sheets by most primers is epoxy primer. So if you’re wanting
to seal that metal off and then apply some body filler, that would probably be the way to go. And to another one of your concerns about applying body filler to the metal, you know, it’s designed to do that. Most of the premium body fillers have zinc in them and that zinc is that provides corrosion protection. So it has zinc in the body filler that is designed to go directly to metal. Now, why can’t you put it over self etch? You know, to tell you
that, I’m really not a, you know, I’m not a scientist or chemist, or don’t know all that. I just know that most
technical data sheets say not to do that, but I really don’t know the reasoning. To me, in my mind, I’m
just thinking that acid’s going to react with the body filler. And either cause an adhesion
problem, like you mentioned, or cause mapping, where
it kind of lifts the edges and you’ll be able to see
what looks like scratches after it’s been painted and everything. That’s not an absolute answer,
that’s just my opinion. But me, you know, it
just says not to do it, so you know, I try to follow
those recommendations. It’s kind of like, you know,
if you drink and drive, you know, you’re going to get arrested, or go to jail or whatever. And we both know that every
time you drink and drive, you’re not going to go to jail, but if you do it long enough, you’re probably going to get caught. And I think this is the same way. I think it’s a good idea just
to follow the recommendations because they do have the paint makers, or whoever is… They test these products and they know how they work best and how they don’t. And you know, I think it’s best just to try to follow those recommendations. So I hope that helps answer your question. What I would do, you know, if I did want to prime that bare metal before I applied body
filler, I’d use epoxy. Thanks for watching and we’ll
talk to you in the next video.


  • sail235aeh

    Good explanation.  Thanks –

  • Simon Neve

    Wow i was not expecting a video replay! Thanks alot!

    just a thougt, would it be possible to say spray with etch primer and then something else ( say primer surfacer ) witch the boddyfiller would have good adheasion to and go about it that way?

    Reason i ask is that i am on a VERY hobby level at the moment and are still locked to 1k products more or less ( cans) and 1k epoxy primer is crap and 2k is either needing a spray setup or is alot more expensive than 1k etch primer cans. So i know the correct way but would it help to go around the rules like that?

  • dannyayo

    It doesn't stick. In my first year as a panel beater I tried it and the filler just came off in lumps.

  • Scott Best

    Great question Simon, I have often wondered that myself, and thanks Donny for the answer, you always help a lot cheers

  • MrElPoderozo1

    Thanks for the video. I am working on my project and I'm trying to get it ready for primer,but what can I do if I do the body work and I think I did a good job but when I sprayed the primer I found some body work imperfections that need more body filler? Can I apply the body filler aver the primer by maybe using some 220 grit sand paper and then apply the filler? Then do the 220 to shape it and 320 before priming again ? Thanks. :0)

  • ojas mehta

    Very Helpful. Have been wondering why you need to do this for a long while now.

  • hunkydude322

    im in my late 50s, still feel good shape etc….   long story short, ive taken auto body repari course at my communtiy college  some yrs back i went  for  3 yr or so, and practice what i learned etc…  at home and painted quite a number of cars etc..,  anyways, im really getting excited in   the collision aspect of it,  im thinikn maybe appling  in near future  to a  good  car dealer  for employment,   what our your thoughts  and opinions,       thanks,    u put out good videos  etc…    see ya.

  • Rammstein Smith

    Great video man

  • Tricky4g63

    the heat from the filler curing on bare metal creates condensation between the filler and metal so even if the panel is dry and clean you still get a small amount of moisture in there and will rust eventually

  • John Willeford

    I used self etch primer to bare metal but i need to apply body filler. What do I do now?

  • SP330Y

    @DIY Auto Body and Paint Hi,after sanding a car down to bare metal,filling then spraying ETCH primer over it, can i go straight to base and clear after that OR is there a stage inbetween ETCH and basecoat that i should do?

  • MrFloppypancakes

    There you have it folks. Don't drink and auto body at the same time

  • The Egg

    I imagine you can just spray a bit of regular primer over it (primer surfacer) then put body filler over that, no? I'm talking about for minor dips.

  • Marc Sorensen

    You dont need self etch primer if you use epoxy primer or SuperBuild.

  • Marc Sorensen

    I painted a 32 Ford that someone else did exactly this…filler over etching primer and the filler bubbled up around the bodylines and peeled off. No bueno

  • Healthy Person

    Great video, cheers mate!

  • Rick Bell

    I have some rust pits which show under the moldings on door of 56. What is best way to solve this issue. Epoxy over pits and then go back and sand and use filler where I ground this area down a bit trying to remove as much rust as possible? Or filler straight on to metal and rust pits and then sand and epoxy over?

  • Fernando Souto

    Firstly, thanks Donnie for the excellent videos! I had an area low at the front of the wheel well, which had some flex to it. The previous owner had filled that area with body filler. On closer inspection, I could see a hairline crack in the filler and when I removed it, underneath was a healthy rust deposit. I see that if the panel has a reasonable amount of flex the filler will crack. I am a novice metal painter but have many years experience with wood. It makes sense to seal the metal surface first and then apply filler.

  • Shaun Savoury

    I etch the bare metal. Also when I am done sanding or shaping a repair there is metal sometimes showing around the repair. I fill then putty. We often have to put the auto out doors awaiting a spot on the floor. putty and filler is porous materials. So what I do is etch the bare metal and coat the whole area filler/putty and all so when it is out doors the weather will not harm it before we can get it in and tape it up to back sand and prime. So when I run it in I lightly sand the etch to make it able to prime. I have been told that your not allowed to etch over body filler and primer by the people in our shop but can't find any info on it anywhere online or from the people making our etch. Is this just them believing what they are saying or are they right? Also I see them etch bare metal around their repairs and the etch can't get on the bare metal at times without getting on filler/putty or even often on sanded clear/paint of they are etching a bare metal spot on a break through to the metal while sanding. Looking forward to your reply. 🙂

  • eric Cervantes

    does anyone know if it's ok to put self etching primer over bondo?

  • Jerry Dicker

    I once made the mistake of spraying polyester primer over etching primer and wow what a mistake had to peel the primer off don't know what chemical reaction went on there but they polyester primer turn the goo underneath and never really dried it was mixed properly etching primer was fully dry before hand spent a week stripping it to start over with epoxy ended up calling ever coat to see wtf and now they put do not apply over etching primer on the label

  • xKAB00Mx

    "im not a scientist or a chemist" …okay >.<

  • Elvis Putnoee

    It will lift off the primer it somehow reacts with it wrong way try to rub harder and it will start to drag and yes f sanding that all of by hand again is a pain so to say

  • sleepe361

    Can you take something down to metal,and etch primer it,then apply paint over etching primer? Or would you have to apply a primer surfacer over the etching primer then paint? Ask because I seen someone painting their brake calipers and they etched primed them,them applied paint.

  • Sleek S

    Can you apply etch primer over bondo?

  • Bob Pendergraft

    So basically self etching primer sucks is what you're saying

  • waynestone Bristol

    Spot on .


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