Projects When to apply body filler?

Hi! Currently doing some body work, and have welded in new metal. I have been scouring the internet and have determined that it is 50/50 when it comes to the point of when to apply body filler. Some say to apply epoxy primer first, and THEN body filler, while others say to apply body filler directly to the metal. My concern is future corrosion. Anyone have any opinions on which is better?

Should I do Epoxy Primer --> Body Filler --> Epoxy Primer --> High Build Primer --> Paint --> Clearcoat

or

Body Filler --> Epoxy Primer --> High Build Primer --> Paint --> Clearcoat

 

I do epoxy primer first. I have for about two decades.

I have not had any issues, nor have my customers.

 

I have applied filler directly to metal for 3 decades.
I have not had any issues, nor have my customers.

Yep, 50/50 so far...

 

Gimpy must be a young buck. Only twenty years . But then again RMR&C is only going back to 1990. Mmmmm

 

I have done both.
No issues either way.

 

I was in a profession that disallowed the working on cars prior to that, and I was not in the US very often.

 

I've done it over metal for forty five years and over epoxy for twenty five or so. Some epoxys work better than others for this, some don't sand and feather well. Epoxy primer is meant for media blasted metal and that's when I prefer to use it.

 

My only concern with bondo type filler is it shrinks.I have seen a number of repairs look fine for the first week,or so,and then I look at it again,and what the hell......It shrunk.In a production body shop theres no time to wait.At home I would apply it,and let it sit for a week,and out gas the solvents then sand it. Im sure some brands are better than others when it comes to this.

 

I do
Body Filler --> High Build Primer --> (single stage acrylic enamel) Paint

and it works fine. But I'm known to be cheap and lazy, and generally only do what is necessary.

Have fun!

 

If you are waiting a week then I would apply over epoxy primer. I say this as a person trying to use common sense, learning from others, not any expertise.

 

Shrinkage!!!
I don’t think I could use cheap filler as a base to build something nice off of. I usually don’t wait,,, if it’s cured it’s cured.
I’ll let the last coat of 2k primer sit a while before final blocking.

Some guys are still using Lacquer primer.

Also,,, if you’re unsure of the products you’re using, get some cups and put in 1/2” worth of material and see how it hardens, cures, shrinks. That’s about the only way you know. Break the test pucks up and see what you’ve got. Make a few and test at published cure time and a week/month later. Some are better than others!

 

Noticed that GM used lead on '60's B pillar, or roof to quarter joints. Keep the metalwork, as close to correct, then thin coat of quality 'fudge'. Sand, primer-surfacer, light sand.

 

Well, I'm just going to throw my $.02 in here and say it depends on your timeframe for completing the paint job on the car. Flash rust will be your enemy here, not the filler.
If it is going to be a while then epoxy prime, if not then metal. You can also see what the directions say on the can.

Sent from my XP8800 using The H.A.M.B. mobile app

 

Epoxy primer first then bondo, done both but the epoxy eliminates the possibility of rust under the bondo.

 

What a unique idea!!.....I'll have to try that!!........Directions,huh?!
6sally6

 

If moisture can reach the base materials, pin holes in weld seams or pin holes in rusty metal etc it will lift any body work no matter the process. I personally media blast, epoxy prime, mud work, polyester primer , sealer, paint.
Also when you see seams or things show up in the baking sun, it’s from a lap welded seam or improper sheetmetal repair. Base metal and repairs need to be able to heat and cool at the same rate or you will see it. I learned this by making this mistake on a project a long time ago now and it will drive you nuts looking at the finished product.

 

Just Some blasting media,
dry ice, soda and vapor are also media

 

Shrinkage.
Applying fillers in thinner coats prevents this. Thick single layers are prone to issues. Better metal work means less fillers.
Proper drying/curing/gas out times help keep shrinkage down with all products.
The only issue I have had with filler on top of epoxy is not allowing the epoxy the cure long enough.
Same issue with using a polyester over epoxy. Epoxy cures slowly by other products standards. Trapped solvents from fresh epoxy will attack itself.

 

I would like for someone to please explain how when bare metal has filler applied over it and it's properly prepped , sanded, primed and painted, how rust can start between the filler and metal?

 

From the back side of the metal..

 

Can you explain that please? Lippy

 

If you do lousy metal work and there are holes in it, then there is a way for moisture to get through. Coating the metal with epoxy primer prevents it from getting rusty.

If you do solid metal work, or don't need to do any hole repair or weld seams, or live in Arizona, then this isn't an issue.

 

Jim, do you mean it won't rust between the metal and epoxy primer? Lippy

 

I've always done metal work as close to perfect as I possibly can, then epoxy prime to prevent surface rust, very minimal mud, block sand as close to perfect as possible, then start the primer/block sanding process. I leave plenty of cure and gas out time between steps. I was taught this is the "correct" way.

In the arena of finish blocking, I wonder if anyone with a lot of experience (or pro's) could chime in on the new age process I see on projects (Foose, Graveyard Cars, etc on TV), where they do a "cake icing" with 12" applicators of the entire car body with a thin coat of (seemingly gallons of) finish filler (Evercoat Rage?). Then they "shape" the entire car with a shitload of block sanding labor. Even after it's done being blocked, the entire surface of the car is an eggshell of mustard colored filler.
Is this really less work than my tried and true old school method of applying minimal filler where needed, blocking and filling pinholes with 3M green spot putty, then progressing to filler primer/block sanding?
I also wonder about the paint life of an "Overhaul" style build that goes from bare metal to final paint in 72 hours w/o any gas out time between processes.

 

I epoxy prime metal, THEN add epoxy primer TO my Bondo, THEN epoxy prime again after my bondo !! This way I don't have to worry about the 50/50 split.

But in all honesty, prep metal properly, then bondo then primer. Since the 80's and no problems. Prep is everything.

 

I think that there is a difference in how well polyester (filler resin) and epoxy resist corrosion. But I live in Arizona, so I'm not an expert on corrosion.

Sent from my Trimline

 

I was assuming the metal that filler was applied over was solid good metal properly sanded with the correct grit scratches for adhesion of the filler. I can sure see problems between the filler and base metal if the metal is in bad condition. And we all have probably seen cracked filler, and that's a sure way to get rust between them.

 

Filler absorbs moisture. Even with epoxy on bare metal, a small pinhole or crack will allow moisture to adsorb into the filler. Causing a potential issue.
Filler goes through a heat when curing. This can attract moisture. Also the time it sits open to the atmosphere will draw in moisture. This is the theory behind using an epoxy under filler.
Same heat/moisture theory applies to lead attracting moisture during the application. Think how when heating something with a torch can cause rust to appear soon after cooling.
The best way to never have filler issues either on bare metal or epoxy is to illuminate as much filler as possible.
Better metal work =s less filler.

A lot depends on where you live.
How long does rust appear on clean bare steel in your shop. How often does a rusty fingerprint show up?
Do you apply filler or primer below or at the dew point? Humidity plays a part in all this.

 

I just have heard over and over how if you don't epoxy prime first over metal moisture can get between them. I have ground a lot of filler off cars in my day and if the metal was solid underneath I have never seen rust between them. Even on cars with very old paint. Was just wondering what others think also. I can sure see epoxy priming a car that has been blasted if it's going to set for a bit before being worked on. Lippy