Technical Body filler over metal vs over epoxy. Interesting video

Both of my Studebaker(s) are still wearing all of the filler that was applied to their bare metal (and some areas well roughed up paint). 50+ years ago. That is good enough evidence for me.
Modern body fillers are supposed to be greatly improved and more durable, but skills and preparation processes have a big impact on durability.
I stick with well prepped bare metal as I am certain that the results will outlast me by decades.

 

Whatever path you choose, please don't use 36-grit for anything.

No shop that I have ever worked in allowed anything past 80-grit on body work.

36-grit gouges will need to be filled with something. If you don't make them, you don't need to fill them.

36-grit is also a really great way of turning already thin metal into a hole that you need to repair.

 

Never liked 36 grit nor understood why folks used it other than grinding welds.

 

It is great for beveling I-beams.

 

Let’s assume the test is 100% valid and as shown, filler should go onto bare metal. But is that the equivalent of using a 3/4in bolt where a 5/16th is needed?

 

Nothings changed.
Flawed test. I’ve never needed anything in painting to survive an impact wrench.

 

And finding the old Lacquer primer swelling up from burying 36/80 grits ravines in the bondo. Yep, mid summer sunlight all day will do this.

 

This doesn’t conclude anything, what kind/brand of epoxy, how many coats, what brand of filler, what is the true pull apart force. The epoxy itself pulled apart, the filler didn’t pull from the epoxy, that could be a number of reasons. Follow the manufacture’s instructions for the material your using and you’ll be good.

 

Especially on pro-grade products, the instructions are the way-to-go.

Manufacturers have done extensive testing, as they don't want their products to be blamed for failure.

 

I have been putting Bondo on bare metal for 50 years no problem.

 

Some things to consider:

Is your metal surface rusty or is it clean/new? If it's rusty you should remove and neutralize the rust and restore the phosphate coating before you apply any filler or primer.

Is the metal surface warm or cold? Cold metal will condense moisture on the surface which will be trapped there under primer or filler, which is the most likely cause of rust forming under filler or primer. Applying primer or filler on cold metal usually means the product will cure from the outside in, not evenly. Later when the metal warms, the uncured product will continue curing and solvents can be trapped in the primer or filler and cause problems later on. If you live in western and southern states with warm temps and low humidity, you probably have very few problems with filler.

If you prefer filler over epoxy you still need to neutralize/prep any rusty metal before priming. Which epoxy is a good choice? Check your epoxy on a test panel. After it is cured, lay a (water) wet rag on it and see if it absorbs moisture and you can also lay a rag with lacquer thinner on it to see if it resists solvent penetration. Body filler contains styrene which is a very powerful solvent. If your primer absorbs water or solvent it is probably not good to use under filler and you could lose adhesion between the primer and filler. Filler over epoxy is still only mechanical adhesion, not chemical. Filler over a soluble substrate (primer) will decrease adhesion and performance.

Mixing the catalyst and applying body filler correctly is critical. Check your shop temperature. If it's cool in the shop (below 75 degrees) the metal on the car will be around 10 degrees cooler. Body filler doesn't cure well or at all below 64 degrees. If your shop is 70 degrees, your panel is around 60 degrees and pre warming (NOT HOT) the panel may be necessary to get a good even cure. If you have temperatures over 95 degrees, it is almost impossible to work filler and get an even cure. If you are not sure of the amount of hardner needed, mix a test batch thoroughly, spread it on test panel, and check the cure time, which should be available from the product tech info.