Just because you can cave and pave using body filler doesn’t mean you should.
The thickness of body filler is a widely debated topic in the body repair industry.
Generally, having more than 3mm of filler in your repair is considered bad practice.
Yet, if you look at the technical data sheets, you’ll see max fill depths of over 5mm.
So the big question is… how thick can body filler be? What are the issues you can face when using too much filler and what can you do to decrease the amount of filler you use?
Table of Contents
How big of a dent can you fill with bondo?
1/4 inch or 5-6mm of filler is the maximum thickness I’d recommend.
For many of the “deep fill body fillers” this is the maximum fill depth stated in their technical data sheets.
While you can get away with using more, it’s considered bad practice.
With the invention of TikTok and other video apps, I’ve seen a lot of things. Filler depths of up to 10mm and even random objects placed in the middle of repairs for the extra build.
You can get away with a lot of poor practices but you should always try to do the job “right”.
Doing the job right means getting the metal as straight as possible before filling it. This will minimize the amount of bondo you’ll have to use.
Thick Body Filler Can Block Radars
Because of ADAS and other types of radars, there are panels on a car that need to be replaced or have limited filler.
Putting filler on the middle of a bumper where a radar sensor sits is going to have an effect on how it works.
In fact, it may not even work at all.
The best way to decipher whether a panel needs to be replaced, refer to manufacturer methods.
You may also find in rare situations where the manufacturer methods tell you to use no more than “x” amount of filler.
Obviously, if you’re working on older vehicles, this isn’t an issue.
Issues you may face when applying too much body filler
There are a lot of old-school techs that will tell you some of the rough stuff they’ve done and gotten away with.
It’s true that a lot of the time, caving and paving won’t go wrong. There are a lot of cars driving around with gallons of filler in their quarters.
But there are just as many reworks pulling up to body shops with cracks in the filler and other defects.
The first issue that you’ll face is pinholes. You must finish with putty or a glaze product to minimize pinholes and scratches.
Secondly, it’s very possible that it will crack. Depending on the location of the filler, it could be almost certain to crack.
If you’re building big edges with body filler or applying it to “flexible” panels, it’s very likely to crack.
There’s nothing more gut-wrenching than building up a car that’s full of filler.
Using harder fillers on panel edges such as metal filler or fiberglass can limit the chances of it cracking, but this isn’t a perfect fix. The same goes for “flexi” fillers.
Finally, If the repair is mint, but the car crashes again, the people will know you’re a hack.
(Providing it’s the same owner that brought it to you to get repaired or bought the car from you).
How To Avoid Using Too Much Filler
Metal finishing isn’t something that’s possible in every repair. In fact, it’s very uncommon.
Getting as close to a metal finish as you can before filling the rest of the repair is considered the best practice.
Depending on the location, metalwork can require specialist tools.
In some extreme cases, you may even need to shrink the panel with a carbon pencil or use other techniques.
Getting the metal work right is not only going to result in you using less filler, make the repair quicker.
My repairs go a lot faster when I’ve got the panel in the right shape.
Providing you’ve got the equipment, it’s best to master metalwork as best you can.
Invest in the right hammers, dollys, dent pullers, porta powers, and pulling systems.
There are some cases where the only option is to either replace a panel or use endless amounts of body filler.
As you become more experienced you’ll know what you can repair, how to repair it, and how long it will take.
Sometimes It’s Better To Replace It
In a body shop, there are a lot of times when a panel is replaced even though it could be saved.
We live in a throwaway world and parts are surprisingly cheap at times.
The cost of a repair often exceeds the cost of replacement.
Replacing a panel also means that it meets manufacturers’ standards and keeps the car “original”.
Just because you can spend 6 hours fixing it, doesn’t mean you should.
It’s all situational.
In a restoration-type job where parts are less common, you may have to repair things that should be replaced.
Or maybe you’re doing a project car on a budget and can’t afford new parts.
This is when spending the time to repair the damage may make more sense.
My Final Thoughts
Always try to get the metalwork as close to a metal finish as possible (within reason).
Staying within the industry and manufacturers’ guidelines for filler thickness is a no-brainer.
There will be times when you need to cave and pave, but it should not become your #1 repair method.
Experience is everything. The more cars you repair and the heavier the damage, you’ll know exactly what you can get away with.