bondo then primer or primer then bondo?

O

old skool

Member
Hello,

I am about to do the body work on my car. I have a number of friends give me some advice on the matter (all do body work for a living). I will be using an epoxy primer. One guy told me to "clean the body and add the epoxy primer then add the bondo" and another said "clean the body add the bondo and then epoxy primer it". I have seen both ideas in the books I am reading. What's your thoughts? Of course am going to get as many "dings" out as I can and use as little bondo as possible. What's the best path to take. I need more input on the matter before I dive in. This will be my first time using epoxy primer.

Thanks for your time.

Marty
 
If you're doing it one peices at a time, rough in your bondo first. But if you're stripping a whole car at once, primer everything so you don't get flash rust, then come back when you work each peice and give it a good sanding with 80 grit or even 36 grit in the direct area the needs the bondo and then you can spread your bondo. You just can't apply bondo over slick metal/primer. It needs a good scratch to grab onto.
 
i never put bondo over paint or primer, just on bare metal for the best adhesion, bondo over paint or bondo will cause either to soften and loose bond to the metal, the heat caused by the bondo as it cures and the chemicals in the bondo will cause the lose of bond to the metal and bondo being harder than the paint or primer makes it very difficult to feather edge the bondo, sanding the bondo will cause more paint or primer to be removed leaving a sharp edge instead of a feather edge
 
After much digging I was able to get to the facts. This included a conversation with the owner of SPI paints.

Here is what you need to know.

Epoxy and bond stick to mechanically etched metal. Each needs a certain type of scratch depending on how the manufacturer made it. This means you use only the type of paper (or sand blasting) for the type of chemical you are putting on the metal. I say this because people are frequently using the wrong grit (too course) paper for bondo type fillers. Too course leaves deep scratches with flat area between, you really want to use the highest number grit paper the manufacturer recommends for best adhesion.

Epoxy during its cure (called recoat window) has an open surface with 'holes' that the next layer can attach to and adhere. So paint or bondo can stick their their 'tentacles' into and then when the epoxy cures it shrinks the 'holes' and locks them down. This is important because once epoxy gets out of its recoat window it is super smooth and does not accept the next layer of paint. All the epoxy I have instructs you to sand and recoat with epoxy with another layer once you are outside the recoat window.

Incidentally, SPI epoxy has a long recoat window which is hobbyist friendly.

The owner of SPI told me the best thing is to epoxy the metal first and then bondo. You will get the best adhesion as epoxy sticks real well.

A word of caution. Epoxy does not cure if there is an acid surface. So if you lay down an etch primer or leave some acid etch behind the epoxy will never cure at the metal interface. So do not be surprised if the paint comes off at the metal to epoxy layer.

So what I have learned is that either bondo or epoxy to metal first if fine so long as you understand the manufacturers rules for application. If you have a choice between listening to the guy with 40 years experience or the manufacturers chemists that designed the product... Listen to the guy with experience and do not go too far away from what the manufacturer tells you to do. I have been lead way astray by 'experienced' body shop guys giving just wrong advice. Remember that you can paint successfully over very rusty untreated metal and it looks good for a while before it falls off.


A for profit shop works faster by laying bondo down then epoxy over it.
A hobbyist may be better off laying down epoxy first to protect the metal from moisture as we do not always have the time to get stuff down quickly.

Epoxy first and bondo during the recoat window is the best adhered.

So long as you follow the rules you will be OK.

Please do not believe me. You can do your own research.
 
I think this probably the BEST answer - and one that is consistent with my own experience.

I alway like to hit that "window" in recoating - if I can. "Keying in" the coatings as we used to say in the diesel shop.

Although most of their painting work was done with rattle cans.

Joe K
 
When I worked in a little shop in the late 70's the Ditzler rep would stop by quite a lot with info.What I remember most is him trying to drill new ways into the heads of 40 year bodymen.We were starting to use a lot of DP40 primer,and he had the experienced guys jumping up and down when he told them to put the DP40 on the metal,and the bodywork over that.That raised a lot of screaming,but when he told the owner that Ditzler would not warranty any product not applied to their specs things changed.The sky didn't fall,no paint blew off in the wind,and the plastic seemed to bind itself to the DP40 better than to the bare metal.Featheredging the plastic into the primer was MUCH easier than blending it into bare metal.There's a dozen cars still around here carrying the same paint 35 years later,some are faded,some have paint blisters,but none have the plastic lifting.
 
Keith True said:
When I worked in a little shop in the late 70's the Ditzler rep would stop by quite a lot with info.What I remember most is him trying to drill new ways into the heads of 40 year bodymen.We were starting to use a lot of DP40 primer,and he had the experienced guys jumping up and down when he told them to put the DP40 on the metal,and the bodywork over that.That raised a lot of screaming,but when he told the owner that Ditzler would not warranty any product not applied to their specs things changed.The sky didn't fall,no paint blew off in the wind,and the plastic seemed to bind itself to the DP40 better than to the bare metal.Featheredging the plastic into the primer was MUCH easier than blending it into bare metal.There's a dozen cars still around here carrying the same paint 35 years later,some are faded,some have paint blisters,but none have the plastic lifting.
Click to expand...

Well said.... I laughed when I read the part that said no paint blew off in the wind. It's funny to me when some people fear even the slightest amount of bondo. When a good quality product is used and applied properly it should outlast the paint job.
 
Mike V is right on in post #3. I started doing bodywork professionally in 1965 and things have changed a lot since then. Back in the day we only applied body filler on top of clean ground metal. Any small spots that the grinder didn't get, we would use a drill with a broken bit to get every speck of paint out of any crevis. Fillers and methods have changed. Actually, water can penetrate right through most body fillers or paints and primers that don';t use a hardener. This is why it is best to prime the bare metal with a suitable primer before appling body filler.
 
when I use plastic (rarely) it is over primer.

My personal preference is to use lead. (yes, I am a young old-school)
 
Purdy Swoft said:
Mike V is right on in post #3. I started doing bodywork professionally in 1965 and things have changed a lot since then. Back in the day we only applied body filler on top of clean ground metal. Any small spots that the grinder didn't get, we would use a drill with a broken bit to get every speck of paint out of any crevis. Fillers and methods have changed. Actually, water can penetrate right through most body fillers or paints and primers that don';t use a hardener. This is why it is best to prime the bare metal with a suitable primer before appling body filler.
Click to expand...
Purdy's "broken bit" trick works GOOD! It was also a MUST, to clean out dark pits, when applying LEAD.
Yesterday I saw a Mustang with Bondo peeling off & the paint under it was NOT EVEN SCUFFED with sandpaper!! When all else fails, READ THE DANG INSTRUCTIONS ON THE CAN!!!!!---Bill W.
 
My Sunbeam Alpine was restored some 15 years ago by a good body shop. They stripped the paint right off and then proceeded to weld and fill with bondo the bits needing repair. Took them three weeks. Then they Primed and painted with Glaserit. Now I have bubbling coming up under the paint and actually sheets of paint and primer falling off. The metal underneath looks pristine apart for a slight almost invisable layer of rust - enough to break the paint adhesion. Prime then Bondo !
 
Purdy and mike are right on Purdy I also started professional as a body man in 1965 also used that broken drill bit still do on occasion remember clipping cars and brazing the floors and gas welding the rockers and the ws posts befor mig and tig were out do u miss the days of metel prep and sanding and buffing that lacquer I am looking for a old lead pot that was used for spraying on lead anything like that around in Alabama land
 
Billy,I've clipped lots of cars and truck cabs to save them in the day. I gas welded them untill they came out with what they called high strength steel and we had to go with mig welding . I still prefer gas welding good metal. Theres not much clipping going on anymore unless its sombody building totals. I clipped a few in the ninties but the insurance companies will total a car before they will allow the car to be clipped . Its risky with unitized bodies if the welds are not good. I always hated buffing out paint and we mostly used lacquer untill the clear coat urethanes came out and we had to change . I don't miss the sanding and metal prep . We did some lead work in the sixties but I have never seen lead sprayed. I don't know of any lead pots like you are looking for in my area.

All of the old guys that I knew that did lead work have passed.
 
My paint/body experience is with boats. When we did bare steel we used a coating called Rustlock, then epoxy, then fairing compound (similar to bondo), then topcoat. Our paint didn't peel off or bubble up, and a lot of those paintjobs were underwater 365 days a year.
 
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