Apply Body Filler Over Epoxy or Metal?

Should body filler be applied over an epoxy primer or over bare metal? This is a question often asked and it quite controversial. In this video, I am going to provide you with my opinions and the opinions of over 100 other people. I found the results surprising and you might too.

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[tylerfraker]

Doing restoration work, if I have to use a fiberglass reinforced filler first, I’ll use that on bare metal. If it’s small enough for regular filler I’ll do epoxy over bare metal first. You have to let the epoxy cure for at least a day or two before you can fill without sanding, but that leaves a short window. If you apply too early the filler will gum the epoxy back up and not let it cure right. Best to let it cure the full window and scuff, but you can do it without sanding if you time it right.

[gpraceworks]

From what I have heard from experienced restoration shops and did a small test on my own, body fillers, do to the activator/hardener heat up once mixed and that heat draws in moisture. Applying DTM could trap moisture in the body filler if you prime over it to soon. Long term that trapped moisture could create a corrosion issue. If you use epoxy first it provides a moisture barrier between the metal and filler to prevent this.

In my test with a heat gun and moisture meter I found the moisture content to be quite high in the body filler, above 18% on average. That is enough to cause the metal to rust over time if trapped.

[justinledoux]

This summer for the first time doing a minor rust repair. I went with the advice of the body supply place to spray epoxy then go to body filler within a 72 hour window after the initial epoxy curing time was surpassed. I used aerosol SprayMax 2K epoxy then Evercoat Rage over top. No sanding of the epoxy. End result after 6 months, looks good. I am hobbiest with little experience which is why this channel is ideal for me to watch and learn! :)

[chrisgray]

Hey I think it all depends on the repair situation....is it production, perfection, or a mix of both...I’ve seen the good the bad and ugly of all repair techniques...but let’s be clear...there are a few things one must consider when this comes up...you need to know what the customers expectations are...I have people say in and day out basically asking me to butcher their car...they get what they want but I have limits...I won’t ever put filler over paint...polyester and peroxide are aggressive...and wherever a feather edge is that doesn’t have 2k hardened materials from substrate to clearcoat and you apply filler over that will map up every time no matter what and faster than you can blink....(oem paint is not hardened from bottom to top)now if you’re restoring a vehicle and you want the best most durable and long lasting finish and money is not in the picture...sandblast...sand w 80 grit apply epoxy and repair as usual...the reason epoxy is recommended...is bc those jobs tend to take months years decades and you don’t want open metal for honestly more than 30 mins...so in the end...someone needs to make the call as to what type of repair your working on...and apply the product that best fits the type of repair...

[robertspicer]

Something missing in the ongoing debate(s) over this subject is one uber important term "mechanical bond" those who think they are getting a chemical or molecular bond going over epoxy are WRONG! It is a mechanical bond! To get the ultimate mechanical bond aggressive abrasion is needed. Problem with epoxy is the maximum dft will have to be exceeded in order to get enough mils of epoxy to aggressively abrade the substrate, in other words the lowest grit of abrasive that can be used is 80/100 grit and thats it. Otherwise you are sanding through your epoxy. liteweight fillers like rage work fine over epoxy but if you are using a heavyweight filler over welds and seams ect. those particular fillers do not feather as well as they do on bare metal and dont bond very well to a lighty sanded epoxy film.

[rogerlarson]

I have used actual Bondo (cheap home depot) directly over metal numerous times and also directly over cheap spray paint on small projects and never had any problems. I live in Arizona where it is dry and always sanded the bare metal down to zero rust. I have never had any problems. I am not saying this is the right way to do it just telling you what i have done. I was always working on my own stuff when i did this. Metal can rust from the back of the panel and if the back isn't sealed it might make a person think the rust was from the Bondo side.

[richdrake]

after trying several different methods on my nova and being unexpectedly delayed on getting a topcoat on it i found that it was better for me to strip the whole thing down to bare metal and use epoxy primer to seal the metal. my epoxy primer states that body filler can be put on top of it.

[Warrior_Resisting_Colonialism]

Well it stands to reason right? An epoxy primer is also a sealer. If you strip down to bare metal, what's more important? To have the metal level or sealed? Sealed of course right? So why not seal it ASAP? If it's possible for rust to form under body filler because body filler is not a sealer, then why risk putting anything down on top of bare metal that is not a sealer? The only reason why you wouldn't want epoxy primer/ sealer first is if body filler wouldn't bond to epoxy primer/ sealer. Since it does bond just as well to it as it does bare metal, then you are only left with two reasons why you would put filler down first. Economics and shoddy craftsmanship/ cutting corners. Maybe in a shop setting, it's more economical to just repair a section by getting it down to bare metal, working out your dent apply filler and then use a 2k primer, base and clear.
If you have the time though, you'd go down to bare metal, prime/ seal. Now your metal is best protected against rust and corrosion and you're not going to get bubble rust in your paint years down the road because you sealed your metal first.

[smitty]

I have had metal rust under bondo
so now i do epoxy let it cure sand and bondo and then 2k prime and block

[chriswalker]

Thanks for this, interesting discussion.
Restoration of Landies Bulkhead, had to use U-Pol Raptor anti corrosive Epoxy Primer first as surface rust forms within a few hours here in Aberdeen Scotland lol.
Then filler over, flatted and more epoxy Primer, to cover any newly bare metal after flatting

[messylaura]

Body filler over steel all the way, then etch bare metal and 2k panel/repair area (proff body / paint in uk)
the things that make bondo fail is holes in the metal letting water get behind it or using crap filler that has no flex in it so it cracks with vibration, i have used 'plastic padding (pp50)' isince 1985 never ever had a problem with it.
best test of flex, leave the plastic spreader with the left over filler on it, when its dry try bending the spreader, see if the filler bends with it or cracks in the middle.
just my life experience in collision repair for what its worth
one of the other things to consider is how many layers of different stuff you are putting down.

[michaelbyrnes]

I’m Aussie born and breed in Australia 🇦🇺 in smash on new plastic cars you don’t need to epoxy primer bare metal repairs as old mate said I’ve found being in the smash industry my whole life From 15 years old I’m now 50 years old new cars have a zink coating on the steel hence the reason you never see rust in new cars or cars there around 10 years old body fillers today have evolved a lot since the 80s high Rezin content too adhere to new car paints aka baked enamel or 2 PACs plus I have worked in higher end smash shops who do use epoxy under there repairs but like I said Ppg epoxy can be speed up by baking it to 60 degrees after two hours you can do your filler repair after it’s baked and as for resto and sand blasting it’s a must to epoxy the car and then filler can be Applied after light sanding or Scotch-Brite epoxy primer is also used wet on wet here in Australia in restoration you have a four hour window before you have to sand it save time on parts like door hinges and underneath bonnets so on so on

[bamf]

hey bro, thanks for taking your time to research this for us... looks like im going filler over epoxy... about to sand with 180, and get busy.... thanks again guys

[peteshelby]

I swear my ocd keeps me awake 24/7 and this is one reason why. Seems There are legit so may answers to this question and you always seem to answer it best. 🙌🏽🤘🏾🤙🏽

[williamgrissom]

I certainly wouldn't purposely sand away epoxy primer at areas where I will apply body filler, which was the 2nd question. Indeed, you can't exactly tell where the body filler will sit since it finds the low spots itself as you wipe it. I always prime bare metal before applying filler since I feel the metal is better protected. Bondo can have tiny voids which might allow moisture to sit against the steel. Just home projects so usually plenty of time for coatings to dry or cure. The better body fillers sound good, especially with zinc if working on a newer vehicle with galvanized body panels. Rustoleum has high-zinc primer spray cans for galvanized metal, but not sure I would want to put filler or other paint over it since when dry it seems a bit flaky, though I do use it as weld-thru primer on the backside of panels.

[yuvegotmale]

Ive done filler over epoxy for over 40 years. early on it was PPGs DP 40. The catalyst in body fillers is what can cause a rust issue. There is a chemical in the catalyst that is the rusting issue.
The way it was explained to me at a PPG training ctr years ago is that epoxy gives you that rust barrier....why take a chance.? How many times have you ground out body filler that was applied over bare steel and seen rusting issues? I have seen it many many times.
In the PPG training they explained that solvent based body filler is going to bond better to a solvent based surface such as epoxy. It seems as though this argument
can go on forever just like Ford vs Chevy.
Its kinda like wypin your butt on a hoola hoop...it just never ends.

[theother]

I go both ways deepening upon the situation. Restoration always epoxy first production usually premium fillers first.

[Supanova]

So many debates over this. Just got off the phone with Tamco they said no filler on bare metal creates heat then condensation on metal and to put over there DTM high build 5310. I do not need epoxy if car is inside all the time it will save you a lot of money just block and top coat your done and still have epoxy properties in the DTM.

[allrightnow]

22+ minutes to present less than 5 minutes worth of information. Good info but learn to be concise.

[DarrylNagy-obso]

Doing restoration yes sanded epoxy. Collision repair 47 years as a tech it's direct to metal. No problems in either case. If water can enter filler it does fail either way. From a rock chip on the outside to unprotected inner surface corrosion can't be stop. Most fillers are porus even lead will get corrosion problems. The reason I expoy a restoration project there in bare metal and flash rust if you don't. Not because of the way the filler is applied..guys in restoration world have a different way of doing stuff. Like filler top to bottom side to side fliiing. . If you ever have to redo one, first thing you think is who butcher this up. But Chinese repro parts is the only way to make them look right. Is that method. I rather do good metal work and sand less filler and make it straight with high build primer, curves in old car bodies reflect light you can feel it but you can't see it. Learn that years ago fixing vw bugs.in the collision industry. Also high summer temperatures and horsepower can take hours of blocking turn a straight body in too a waving parade car in no time. Filler s and metal move at different rates. As do primers and paints. They only stay straight for so long unless you don't drive them.