Is Bondo waterproof? Can Bondo Get Wet? The short answer is no.
Bondo and glazing putty are very porous.
If you’ve repaired a car and the Bondo has been rained on it’s likely to fail over time.
While you could get away with a slight drizzle, any major rainfall will cause issues.
Moisture trapped behind the Bondo will begin to cause rust and will begin to show through the repair.
A lot of the time the repair will not fail straight away.
Instead, once you’ve painted the car and you or the customer drives it around for a while, they’ll start to show.
This means you’re going to be repairing the car twice.
In this article, I’m going to teach you how to keep your repairs from getting wet and what to do if they do get wet.
What happens when Bondo gets rained on
When you’ve left your repair outside and it’s been rained on there are 2 common things that will happen.
The first and most obvious issue is rust.
Any moisture that’s trapped behind the filler will begin to cause rust to the metal.
If it’s a very small bit of moisture, it could take years until it causes problems.
At the start, it’ll be surface rust, but over the years it could spiral out of control.
Rust is the cancer of the car world the only way to fix it properly is to cut it out and remove it.
Secondly, you could encounter moisture bubbles.
Depending on how much water is in the filler, it could start to bubble.
I’ve seen cars come out of a bake cycle with the filler already bubbling and lifting.
For a shop, this is ideal, since they can rectify it before the customer even knows.
However, if it begins to bubble and lift over time, the customer will know and the shop will get a bad rep.
If you’re doing a DIY project or a car for yourself, sending it and coming back to repair it again when it fails is fine.
It’s just going to create a lot more work the second time around. As long as you keep the car, there are no morality issues.
How Long Can Bondo Be Left Unpainted
Bondo can be left unpainted, but it will be susceptible to the elements.
There is moisture in the air and it could get into your repair.
It’s not going to cause any major issues as it’s not really enough to be significant.
If you’ve applied epoxy primer before filling the car, it’ll be fine to sit in a workshop forever.
If not, your best option is to prime it or paint it. Especially if it’s going to sit around for a prolonged period.
How To Prevent Bondo From Getting Wet
Sometimes there are reasons why you can’t progress a job forward.
I’ve seen cars sit around a shop for ages before getting painted.
This is normally because of back order parts but in some cases, customers not paying bills.
Here are 3 things you can do to stop your repairs from getting wet or any other environmental effects damaging the car.
Store it correctly
I’m not delusional. Having a lot of storage space is a privilege that not everyone has.
Some people will be working in a 1 car garage or even working outside. So for those people, this recommendation is irrelevant.
However, for those with enough space, you should always store the car inside.
This way it’s not susceptible to any rain.
You wouldn’t even need to prime it either, just let it sit in filler.
This is the best way if you own a big shop or space. Obviously, not everyone does.
Primer
If you don’t own a big shop or sheltered area where you can put a car, the next best thing is primer.
Just like filler, primer is porous.
It doesn’t actually protect the car from rain like everyone seems to think.
However, putting the car in primer is a little bit better than bare filler.
It’ll protect it slightly and has slightly less chance of moisture being trapped.
If the car is sat in heavy rainfall, especially overnight you’ll probably need to redo the repair anyway.
The best way would be to put a top coat on it, but that’d mean sanding and prepping the panel twice.
I’ve seen cars driven around in primer forever before being painted and the repairs have lasted a long time.
It’s probably just luck of the draw.
Cover it
If you can’t store a car, the next best thing is going to be to prime it and then cover it.
Having a waterproof car cover that sits tight to the car and covers the repair would be best.
However, you could always get duct tape or masking tape with some painters sheeting and cover it up.
Only put the tape on panels you’re painting or blending as the wet or hot tape could cause defects to the paintwork.
If this is a common occurrence in your shop it may be worth investing in a couple of car covers.
Or if you’re someone with a small unit and outside parking that constantly rotates between cars, you definitely need car covers.
The One Exception
Plastic!
If you’ve got a car that’s in bondo already but all the repairs are on plastic it’s perfectly fine to get the filler wet.
Ideally you’d take them off the car and store them inside. Assuming they’re bumpers or mouldings.
This is the best practice but there are a lot of shops or DIYers that don’t.
If you do get your filler wet, dry it out before painting.
The filler will still retain water, it just wont rust the panel since plastic cannot rust.
Can You Wet Sand Bondo?
Wet sanding bondo is a surefire way to make the car rust.
The bondo will retain the water that you used to wet sand and begin to rust underneath.
Moisture under the bondo could also result in bubbling over time or when the bondo is baked.
Because of this, I’d seriously recommend avoiding wet sanding bondo.
If you’re using a good sandpaper, the best body filler and the best auto body sanding blocks, there is no need to wet sand. You’ll be able to achieve the same, if not better results without wet sanding.
And even if you think it is going to give you better results, is it worth risking having to re-do the repair or have to repair rust when the car comes back?
What to do if your bondo gets wet
If you’ve let your bondo get rained on there’s only one way to guarantee the quality of the repair.
This is to redo-it. Remove the bondo from the car and start again.
Take the filler back to bare metal and make sure there’s no surface rust beginning to show.
This isn’t a huge deal if it’s a minor repair. But it’s still lost time in a job that’s all about efficiency.
The big pain in the ass is if it’s on a big and complex repair.
Just think about all the effort you’d have to go through, removing all the filler and redoing all the lines.
Screw that! better to prevent it from getting wet in the first place.