The quick answer is yes, you can put Bondo over paint, but only certain ones.
I see it every day in a bustling body shop and hasn’t caused any issues in years.
That said, applying Bondo over paint is one of the most debated topics in auto body.
Any old-school technician will tell you to take the repair back to bare metal and feather edge the paint.
Whereas newer techs will know no different.
It’s still taught in trade schools and body shops around the world, that bare metal is the best practice.
This is true, it’s the best practice but it’s not always necessary.
Keep reading to find out which products go over paint and any possible issues you may face.
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Bondo & Body Filler Products that can go over paint
Not every bondo is made to go over paint. It’s only certain ones.
As the industry and technology advance, I believe we’ll see more and more fillers that can go over paint.
At the time of writing this article, it’s only glaze products that can go over paint.
These products are very similar to a painter’s putty and can be used for spot repairs and light skims.
Traditional deep fill fillers, fiberglass, and bondo still need a bare metal finish.
Because of the thickness of the traditional fillers, taking them over the paint would probably leave a very obvious high.
For the best finish, you should repair the damaged area with normal filler.
Once the repair is 99% there, finish with a skim of glaze.
Take this skim over the feathered paint and rub it with 180-320 grit paper.
This will help to fix any defects and imperfections and will also lose any paint edges.
If you have any doubts about a product being applied over paint, check the datasheet online. It takes a lot less time to read the data sheet than it does to redo a repair.
What Can Go Wrong When Applying Bondo Over Paint?
The main reason why repairs come out poor is preparation.
Poor or incorrect prep can lead to a whole host of problems.
The main issue you’ll have when applying filler over paint is not keying the surface first.
Sometimes, when sanding panels you can miss areas.
If you apply products onto paint that isn’t sanded properly the Bondo will fall off when rubbed.
The other issue you’ll face is a lot less common. However, if you work on older cars that have already been painted, it could be an issue.
In the technical data, both Evercoat and u-pol suggest that they’re products can only be applied over OEM paint.
Similar to a house, your repair has a foundation and starts at the bottom.
If you apply filler over a car that’s been repainted poorly, it could flake off over time.
Most of the time you’ll be fine, but if a car looks to have poor quality repairs on it, best take it back to metal.
How To Tell If It’s Unoriginal Paint
While there are definitely shops and painters that are amazing at their job. There are also a bunch of hacks.
The easiest way to tell whether a car has been poorly repaired is by using your eyes.
Most of the time if the paint has been poorly prepped or repaired, there will be telltale signs.
It’s not going to be the best finish and could be cloudy, full of orange peel, or just a poor match overall.
The other way would be to check the thickness with a paint depth gauge.
Any major difference in depth is likely to mean that panel has been repainted.
Over time, if you’re in the trade, you’ll be able to tell a poor repair a mile off.
What benefits does applying filler over paint have?
The main benefit of putting filler over paint is efficiency.
If you’re in a bonus shop and need to get through the work quickly, spending the time taking a panel back to metal adds up.
With products such as dolphin glaze, you fill scratches and minor dents in a matter of minutes.
I also believe that it gives a more continuous finish.
Instead of going from filler to metal, to feather-edged paint, you have one continuous layer of bondo.
This also helps to minimize any imperfections and scratches on the paint.
Finally, and probably most importantly.
It leaves the original coatings untouched.
When made in the factory, cars are acid-dipped and zinc coated. This protects the sheet metal and prevents rusting over time.
If you take a repair all the way back to metal, you remove this coating.
By filling over prepped paint, all of the original coatings are still intact.
This is especially important because we want to keep the car as original as possible.
Body Filler Over Primer
Body filler or bondo over primer isn’t something I’m too familiar with.
It’s not something that I do and isn’t really used in a body shop with a quick turnaround.
Applying body filler over epoxy primer is a legit repair method that’s commonly used. More so in restoration.
If you’ve got a car that’s been paint stripped and taken back to metal, you should epoxy it to try and prevent corrosion.
I’ve also seen some shops with poor storage epoxy prime and fill cars before throwing them outside.
There are definitely situations where this comes in handy.
However, when doing the job the same day, it’s not something that’s necessary.
Again, this is my opinion. If you speak to an old school resto-nut, they may say otherwise.
I may be tempting fate, but I’m from a shop where everyone uses glaze over paint. Here are the two products I’d recommend.
I’ve not seen anything go wrong in the many years that this has been done.
A lot of the old-school technicians have even started to come around to the idea.
It definitely works, but there is a margin for error.
I understand why taking repairs back to bare metal is taught and should be continued to be taught.
There are still a lot of repairs where you’ll need to take them back to bare metal, especially in the beginning.
You cannot miracle-pull a painted panel. You’ll also not be able to use deep-fill products over paint.
Bigger repairs still require traditional methods. However, car park dings and small creases can be sorted with glaze or PDR.
There is more than one way to do the job, just find what works for you and keep at it.
It is possible that one or two jobs in a thousand will come back and bite you in the ass.
That said, it happens, and not every job is going to go perfectly.
As the trade develops, more and more products are going to change the way that work is done. You just have to decide whats right and whats not.