Several do, I prefer bare metal. If you do put it over primer be sure you are using epoxy. Those who do put it over epoxy probably do it so that the metal does not get rust before they get it on. I live in a dry climate and have left pieces for several months without rust. Some people apparently think bondo adheres to epoxy better than to bare metal -- I don't know. In 14 years I have lost none.
 
Epoxy yes --metal etching primer No. Note I have 12 days of training in paintwork. lol { But I read the instructions alot]
 
I agree with you 100%, I like bare metal, I even give it a light scaling with sand blaster over grinder or sanding marks right before the bondo. Done body work before there was even bondo. I would also light hit the bondo with small blaster held way back to open up pin holes before priming.
 
If you are using epoxy primer it's best to prime first and then bondo. The primer provides a better rust preventive that way. Otherwise I would prime over the bondo.
 
The reason that those of us who promote bondo over epoxy do so is that bondo is porous and will not only absorb water but water will go through to the metal underneath and may never completely dry out. This is only a problem if you wet sand your primer surfacer (which is the only right way to do it) because inevitably you will break through some places where it is high and when you do you now have bare bondo and water at same time. Block sanding with water is how one gets things perfectly flat. If you have no need to block sand your primer or if it just needs dry scuffed to paint then primer under bondo isn't needed but still good corrosian insurance. This is coming from an auto body/paint tech meaning our work goes out in salt/water/sun/stone chips etc for years so for a barn kept hobby tractor it may well be overkill. Just explaining the reasons and pros/cons. You decide.
 
I did body work 44 yrs for a living, I worked strickley by commission and made good money at it and I had to warranty my work and if a job came back I ate it! The reason I don't want bondo over primer or paint is adhesion. I have redone other places work where the bondo let go. I only wet sand the final sanding, after all is stright and flat.
 

If you have any rust pits in the metal give it an acid wash before applying filler or you will be leaving rust to grow under it. It will take the rust years to push the filler up so it depends on how long you want it to last.
 
First let me clear I am NOT a pro - typical barn hack here. But your question was CAN you, and I'm comfortable answering that with yes you can.

It may or may not be the "proper" thing to do, but I've done it for years with no problems.

I only use epoxy primer.

I scuff the primer good with a pad before applying the bondo (don't go crazy epoxy can get a little gummy if you try to sand it). I then do surfacer/sanding primer over the bondo.

Personally - with my untrained eye - I just find it easier to do the filling step on a nice clean, smoothly primed surface. I just have a better "feel" for a surface once it's been primed vs. bare metal. Maybe all I need is better lighting, but that's just been my experience.

I'd also add that I rarely use a lot of it. Spot putty more than anything. I usually hammer out the metal to get it as close to perfect as I can to minimize the use of the stuff. Personally I think it's worth that effort. I'm not sure if it's right to say this, but I feel that there's less "structural" demand on the bondo that way. So adhesion - while always a concern - isn't a worry.

If you don't, and you're say... filling a softball sized crater on a panel that flexes... I'd probably go directly to the metal with it.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top