Have you ever applied filler only for it never to harden? I have.
There’s nothing worse than applying a lot of filler over a big repair and having it never harden.
Most of the time this is because you’ve not put a hardener in it.
New apprentices and helpers are likely to do this at least once in their careers.
Everyone has off days too. I’ve still seen experienced technicians get distracted and forget to do it.
In this article, I will tell you how to make bondo dry faster and what to do if it’s not drying.
Table of Contents
How long does Bondo take to dry
When mixed and applied correctly most Bondo will dry within 15-20 minutes.
You’ll find that this varies between products.
Body fillers are often applied thicker and take longer to dry.
Whereas glazes are often applied in thinner coats and dry much more quickly.
You can check the technical data sheet of each product.
In the TDS you can find out how long they’ll take to gel and how long until you can rub them.
According to the TDS, Rage Ultra is the fastest-drying body filler on the market. Drying in under 10 minutes in optimal conditions
How long does Bondo take to dry without hardener?
If you’ve applied Bondo without a hardener, it will never dry.
Most modern Bondo and body fillers are 2 part products. Meaning they require the hardener or catalyst to change and harden.
Without the second part, all you have is wet Bondo.
It’s like making egg-fried rice without the rice. All you have is egg.
The only way to fix this is to apply a hardener.
You can either take the filler off completely or add some to the filler and mix it on the car.
How To Speed Up The Bondo Drying Process
In a body shop where efficiency is rewarded, you’re going to want to speed up the repair process.
Repairing cars and filler work is normally where a technician makes his money since R&I times are so bad.
If you’re not in a shop, there are still benefits to the Bondo drying faster.
Nobody likes standing around and wasting time, do they?
I’m going to preface this by saying you shouldn’t actually try and artificially improve drying times.
This can cause defects to happen and actually result in taking longer.
However, I’m aware people do it and have done it plenty myself.
Turn The Heat Up
The main reason filler takes a long time to harden is due to the temperature.
You can see the difference when you apply Bondo in the summer versus the winter.
In the summer you’ll often have a couple of minutes to work with the filler, whereas in the winter it can be 10+ minutes.
The best way to solve this is to work in a climate-controlled garage.
The TDS even suggest that higher temperatures mean faster drying times (within reason).
Once the temperature starts getting too high, you’ll encounter other issues.
Not everyone has the luxury of a temperature controlled garage.
The next best thing is a heat lamp or heat gun.
Using a heat gun will make the bondo dry a lot faster.
There are downsides though.
Over-heating the bondo can lead to it burning and flaking off.
You may also find that the bondo drys on the top, but not underneath.
Manufacturers don’t recommend this method.
That said, it does work and is widely used in the trade.
Apply more hardener
Applying more hardener will result in bondo drying quicker.
This is more risky than heating the bondo and can lead to more serious issues down the line.
The mixing ratios are recommended by manufacturers for a reason.
Some of them do offer a % range, but, even then it’s normally 2-4%.
Putting too much hardener in bondo will decrease the amount of time you have to apply it.
It can also lead to it bleeding through paint after repair which is a very costly defect.
I don’t believe anybody is weighing out their bondo ratios.
Sometimes you’ll put a bit more in and other times you’ll put a little less in.
Over time, you’ll learn what you can get away with. Auto body is a trade that’s gets much easier with experience.
Different Hardeners Change Curing Times
I’m not recommending using hardeners other than the one that’s supplied with the bondo.
However, I have noticed with experience that different hardeners will change curing times.
Using the blue cream hardener from a body filler on dolphin glaze would decrease the cure time.
Again, this isn’t something that I recommend doing. The catalysts provided with each product are used for a reason.
Mixing up fillers and hardeners is likely to cause problems at some point.
It works until it doesn’t.
Mix it for longer
Mixing the bondo for longer than you normally would can speed up drying times.
As you fold and mix the filler, more air passes through it.
Air is one of the main factors that accelerates the body filler hardening.
This will significantly decrease the time you have to apply the body filler.
To do this, you have to be confident in your skimming abilities.
You need to be able to get a good skim with the first pass and clean up, otherwise it could dry with big edges and too much filler on it.
Forgot To Put Hardener In Bondo, What To Do?
If you’ve applied bondo but its taking forever to dry, you have a couple of options.
The first option is to wait.
If you’ve applied even 1% hardener and mixed it properly, it will dry over time.
You can use the methods above to speed up the process.
This will still take forever and is probably best to remove it for speed and to prevent defects.
The second option is to remove it and re-apply.
This goes for body filler without hardener as well.
Use a spreader to scrape off as much body filler as possible before using panel wipe and rags to remove the rest.
There will be bondo left over on the panel, which can be cleaned up with a DA sander or another type of cleaning wheel.
Finally, you can try to apply hardener on the repair.
Scrape up the wet or un-catalysed bondo into a small area and apply hardener too it.
Then proceed to mix it on the car, the same way you would on a mixing board.
This is easy if you’ve got bondo with no hardener in it at all.
But, if you’ve got some hardener in it, it’s a race against the clock.
There’s a good chance that it’ll begin to dry and be a lot harder to remove.
Again, you’ve got to experience these situations and you’ll learn what to do over time.
Defective Bondo
It’s very unlikely that you’ve got defective bondo, but it is possible.
If you’ve done everything on this list, including applying the correct amount of hardener, you’ve got a defective product.
Make sure to check the expiry date on the tin.
I’ve seen Bondo used well beyond the expiry date but that’s my first thought.
A lot of educational centers are donated out of date Bondo and they can still provide a good finish.
The only thing you can do to sort defective bondo is replace it.
If it’s within its expiry date, you could probably contact the manufacturer and get a free replacement. Bondo is starting to get really expensive so any money saved is great.