My camaro has spots of surface rust where the previous owner tried removing some of the paint. My question is, when you sand the rust away do you put the body filler right on top then prime it or do you prime the metal then fill it then come back with more primer. The last thing I want to happen is see bubble in the paint down the road. What is the proper way to go about this to prevent any future problems?

 

Well the first thing to do is be 100% certain you've got all the rust, or nothing will stop it from coming back and bubbling.
I've talked to numerous bodymen with years of experience, and I'd say it's 50-50 as to what the responses were on bare metal vs. epoxy primed metal, prior to applying bondo. I can personally say I've tried it both ways myself, and not had an issue either way also.
I used to be of the mindset that it always went under the primer, so I did that, but now I get things hammered out as close as possible and do the bondo on the bare metal. Then once I've epoxy primed the body I go back over and do any touchup filler right over the epoxy primer.
Since the experts are divided, I can't say one is better or worse, and I'd bet we get some conflicting opinions to your question here also.

 

Just like Val,i have heard alot of argument as to which way is right,so i would say which ever way you want to do it is the way to go haha.Like mentioned make sure you get the rust completely or it will come back to haunt you in the future.One thing you can do to insure it is gone,is after it looks like it's all gone,go over the area with some type of rust nuetralizer.There are a few kinds you can get at the parts stores,i forget the names,but you can also use i think it's called muratic acid which you can get at lowes or home depot.The nuetralizer before any body filler or primer goes on will make sure that the rust that you dont see,wont come back in a few years.

 

muriatic acid is nasty stuff, but works great. it will eat any left over rust right off (its also really good to clean concrete floors). if you use it make sure you have a respirator and some gloves. also use sodium bicarbonate to neutralize it. its a pool chemical so look in the pool department or at pool and spa stores.

 

Normally, you'd want to put the body filler over bare metal. But in your (pre-rusted) case, I'd want the metal to be first treated with something made for protecting the metal.
Bondo isn't a metal protector/rust stopper.
First, treat the rusted metal with something along these lines: Rust Remover - Eastwood Fast Etch
Then prime with a self etching primer, which is the kind of primer you want to use on bare metal: GraySelf Etch Primer Aerosol 16 oz - Aerosol Etch Primer - Etching Primers

 

This in not black magic. You must remove the rust, just because you can not see it does not mean it is not there. In fact IF you can see bare unprotected metal I can assure you the oxidation process has begun. The process is instant.

The steps of fix oxidation are simple.

Remove most of the rust (grind)

Treat area with a Rust convert (AKA rust killer) this will turn ferrous oxide to black oxide

Prime area with a etching primer (This has acid that will kill flash rust, and should be used ANY time metal is exposed)

Seal it (its time for that epoxy primer)


Do what ever else is needed. If you skip a step the rust will be back.

FYI bondo is NOT a sealer, in fact it will wick moisture to the cold metal. If you put bondo on bare metal then paint on top of it the paint becomes the sealer but a chip in the paint can destory the seal and allow the rust to return. Most primers are NOT sealers make sure you check with the manufacture. Some primers that that are sealers have a window that they will need top coated by, again check with the manufature.

I was never a professional bodyman but I sat through the classes as it was part of my Automotive Tech degree. As a manufacturing engineer I have to deal with rust a lot. The stuff I make can not have rust popping up ten years from now.

Rust converter
Eastwood Rust Converter | Rust Converters | Auto Rust Converter

If you are just doing a small area most hardware stores have this
Permatex 81849 Rust Treatment (Body Filler Compatible)- 16 oz a

Etching primer
Gray Self Etch Primer Quart

Also note ther are some other ways of getting rid of rust but most are too hard to do with a assembled car body.

 

I've used the "rust converter" type stuff before (not Eastwood) with lackluster results; the rust eventually came back. And that was with using my high speed right angle wire wheel first to get the metal "clean". From past experience, I'm not a firm believer of the products that convert rust to a paintable surface, for the simple fact that rust (iron oxide) is no longer part of the metal it formed from. The slightest bit of rust in a pit, and it'll be back. That's why I recommended the rust remover.

 

The best thing to do is read the P sheet for your primer. They dont really make etch primer anymore. A good epoxy from SPI, DuPont, or PPG will seal your metal. I too have done over and under. I read on SPI's forum they prefer to have the filler over. Their reasoning is that the epoxy creates a barrier. Apply filler under it and it may have wicked in humidity and then gets trapped there. if the prep is done correct the filler wont come off the epoxy.

Also use a good filler. Might as well spead poop on your car if your actually using Bondo brand. That stuff id rock hard and sands like your sanding a brick. Anything from the Evercoat line is good. Rage, Rage Gold, Z Grip are all good names. Rage sands very easy but some think its too soft. I use Z Grip on my car. Sands awesome. I can knock down with 80g if I have to. A good catalyzed spot puddy comes in handy too!

 

The only way the rust comes back is if you do not follow the steps. Black oxide is inert. it will not grow, it is the same thing your allen wrenchs are coated it. Remember the rust converter are for the very small pits only, I will do several passes with a scotch pad on my 7" sander before wiping the pannel down with the converter. Remember the rust itself contain oxygen and moisture so even IF you seal the surface everything the metal needs to rust is under the sealer. If you can feel the rust with your finder you need another grinder/scotch bright pass.




You saw I posted a link to buy etching primer? Sealing up steel that has already started the oxidation process could back fire on you.

Self Etching Primer vs. Epoxy Primers - Autos.com

 

Eh, I'd still rather use the solution that removes the rust as opposed to converting it.
And yeah, who doesn't make etch primer anymore? You can also get it in an aerosol like the one I posted above.