Bondo question

[truckinman466]

I just watched a video from Eastwood on plastic fillers. They applied it to bare metal. I thought you were supposed to primer seal it first. Which is correct?

[Cuda Cody]

Some plastic fillers do say they can be applied over bare metal, but most prefer to put it over primer. If it's my car, I'm using epoxy primer first.

[torredcuda]

Some do filler work over primer some don`t. I have always put filler over bare metal just using a quick metal prep first and never had an issue. I have heard the newer lead free epoxy is not as good to fill over as the previuos lead version but have no personal experience.

[RUNCHARGER]

I've done it both ways as well and haven't had a problem, epoxy first is the recommended method though.

[71vert340]

 I've done it both ways but prefer a self etching primer at least. I have a R/T hood on the 71 Challenger hardtop that someone had put a filler skim coat over the entire hood. I removed the filler and found small rust pits over the surface in many areas. Treated the hood and primered it. Just used filler where needed. Still no problems 16 years later.
Terry

[Mopar5]

I have to respectfully disagree with putting filler over epoxy or primer. I always go over bare steel that has 40-80 grit sanding scratch.The filler gets its grip via a mechanical bond with the initial ground areas.thus your very first coat should be pressed into that scratch very well.going over smooth primer or epoxy is not a better grip. Always check the tds or technical data sheet for your product they are easily found online and will tell you how to apply.

[kawahonda]

In 2020, the leading thought is to use epoxy primer first, like Cody says.

I've many times in the past put filler directly on bare metal....and as far as I know (sold the work many years ago) it has held up just fine.

I think both answers are correct, but there is definitely sound thought as to why putting it over epoxy is the "ultimate." It's an extra layer of protection. One benefit that it adds is that many epoxies (if not all?) have some sheen to them, so you can easily spot areas that need filler after spraying. Sometimes with bare metal, it's hard to spot every single imperfection, even if you have a good hand. Obviously block-sanding will give it all away, but we all know that filler is best handled prior to block sanding high-build.

The negative to the epoxy-first method, is that you tend to use more material (epoxy). Epoxy is used as a standard for a sealer coat before your base (so you can uniform the color and hide any last bare metal spots that showed up during block sanding--or wet sanding) so you'll likely have some in your cabinet regardless (or you should). If you epoxy first, you will just have to use more of it, which means more material and cost. The benefits still outweigh the costs I would argue.

I tend to prefer epoxy over etching because etching requires an additional sealer before top coating since it is acid-based. It's just a less flexible base-primer, although I have no large qualms with it. Epoxy has more uses.

I have yet to see epoxy primer in a spray can. Sometimes when working on very small items (think motorcycle parts), it's really annoying to break out 2k paint and even a detail HVLP gun to spray a tablespoon worth, so sometimes it may make sense to use a good quality 1k rattle can of epoxy before filling. Let me know if you guys spot a good quality epoxy rattle can, because I think having one in the cabinet is worth it!

The other benefit to using epoxy is that you can immediately prep the part for primer after metal work. If you are in a humid environment, this has many obviously benefits.

[Rbob]

I am a fan of epoxy first on restoration work, if you have to apply a second coat of bondo you will be applying partially over bare metal (unless you re-prime ughh.) then just spot prime with the epoxy primer before applying 2-k primer. 

Just keep in mind that bondo uses H2o2 hardener (H2o is water) so use it sparingly.

I use to love PPG DP40 but its no longer available.  The best I have found since is Southern Polyurethanes Epoxy Primer.  Apply bondo direct to the epoxy the next day, you can block the filler and the primer feathers beautifully. The tech sheet on epoxy starts on page 12: https://9bebbfda-9708-47ec-91b7-e2bc3701bf48.filesusr.com/ugd/8ced3e_355f0e9282e54a26bafda5cfb80cd11b.pdf

And great info for someone attempting their own paint job: https://www.southernpolyurethanes.com/perfectpaintjob

[ek3]

the correct/best  primer will have a type of zinc or chromate or other rust inhibitor in it and should be applied directly to the metal where it does its job. primer applied over filler does not contact metal. also, a second benefit is they are so hard when dry that they keep moisture from getting to the metal which = no rust.  if you apply any filler or surface builder over it, you need to  sand the surface first to provide a "profile" for it and, the same goes for the bare metal before epoxy. i have always felt that a tie coat over epoxy  before it cures to hard will insure long term adhesion of all topcoats. the tie coat is easy to sand compared to the epoxy. each primer has a purpose ..

[SRT392]

kawahonda,   check out Eastwood's catalyzed aerosol primer in a can..  $24-$26.  pop the bottom which breaks the activator inside and shake/mix it.  Disadvantage is once activated, you have to use it, and then toss it. So, what's $24 compared to fire up the compressor, get the gun out, get the reducers/thinners out, prep the gun, mix the primmer, wait the appropriate amount of time, spray the area, toss the excess, then clean the gun, put it all away.  I do keep a can around just in case i get in a real hurry but for the most part i'll spend the time (at least an hour) and mix it up. 

[kawahonda]

Yeah--that's my fear which is a two-part can.

My local shop has those already. But like you said, once you pop the top, it's all over!

I think we've confirmed that Epoxy cannot exist in 1k. Etching can however....

I have plenty of epoxy and no jobs currently, so I'm golden. But who knows, right? I may need to spray a 1/2 cup's worth, and it's a annoying. But then again, it's winter! 

[RUNCHARGER]

I use spray can "epoxy"  primers but only to sand them off later. I will coat the metal to seal it from the atmosphere but I would never topcoat over them. Maybe it's okay and I see guys on TV or Youtube doing it but I don't think it's worth taking a chance.

[jimynick]

I've tried it both ways and feel that there's better adhesion with the bondo on bare metal. That's not a slap at epoxy either, as I think it's one of the best things since sliced bread.