Yes, Bondo can be used on plastic. Almost all Bondo will work on plastic but some are more suited than others.
Using flexible fillers that are designed for plastic surfaces is the best practice.
Different substrates have different properties and act differently. So it’s best to use the right bondo, to prevent defects from occurring.
The Correct Body Filler For Plastic Bumpers
The body filler used on plastic bumpers is different from the Bondo used on metal. You’ll need a plastic bumper filler such as U-Pol Stronghold For Plastics or Evercoat Polyflex.
Most of the plastic parts on a car are thermoplastic. This means it’s possible to reshape the plastic.
The most common piece of plastic you’ll be repairing on a car is a bumper. They’re designed to flex and absorb impact.
Because of this, you need to use plastic bondo that has some flex.
You can also find thermoset plastics on cars. Thermoset plastics are hard, brittle, and aren’t reshapeable. Instead, they break or snap.
(Traditional bondo once fully cured is also thermoset). If you try to flex a traditional Bondo once it has cured, it will snap and crack.
How To Know Which Bondo Is Fine To Use On Plastic
Most plastic bumper fillers will have it in their name. They’ll say “plastic filler”, “flexi filler” or something along those lines.
However, as a general rule of thumb, before you use a product for the first time you should look up two things.
The Material Safety Datasheet (MSDS) and the Technical Data Sheet (TDS).
These are two documents that the manufacturer is required to produce.
The MSDS tells you the risks when using the product and the TDS tells you how to use it.
Reading the technical data sheet of each filler will help you understand the surfaces it can be applied. How to apply it and so on.
You’ll also find that some plastic fillers recommend the use of plastic adhesion promoters for best results. Some are even required and are part of a 3-part system, instead of the the traditional 2-part bondo and hardener system.
What Happens When Using The Wrong Bondo on Plastic Bumpers
Using the wrong products for the job will result in defects. These become obvious at various different stages of repair.
Bondo doesn’t adhere
Bondo and body fillers will stick to a number of surfaces. Almost all of them will stick to plastic providing it’s prepped properly.
If you haven’t taken time to read the TDS of the filler you’re using, don’t be surprised if it starts flaking off.
For best adhesion, you should use a “plastic adhesion promoter” and plastic bumper filler.
This will minimize the Bondo from falling off when you’re shaping it, or flaking off when painted. It also increases the chance of your repair outlasting the car itself, which means no reworks or upset customers.
Bondo Cracks when flexed
This is by far the worst thing that can happen (in my opinion).
Traditional, thermoset fillers are prone to cracking under flexion.
If you’ve repaired a bumper in an area that’s likely to flex using normal bondo, it will crack at some point.
The worst thing is when you go to build up the bumper to put it on the car and you’ve cracked it yourself.
If you’ve ever spent time repairing a bumper, got the repair perfect, had it painted, and went to refit it, you may know this pain.
And as a result, you’ll also have a painter and your bosses on your ass.
Another scenario would be that it holds up and leaves the shop. However, as it’s driven the bumps and impact of the road, the filler cracks.
If your shop has a warranty on repairs, the customer will be entitled to a free repair.
Using plastic fillers will decrease the odds of both of these things happening.
If you do it right the first time you won’t need to do it again.
Best Bondo For Plastic Bumpers
The best bondo for plastic bumpers is one that’s flexible and made for plastic repairs.
I personally use U-Pol Stronghold 706-1 as a plastic body filler and Evercoat Poly Glaze as a plastic glaze. However, most of the fillers are very similar. There’s no harm in trying other ones out.
If you find yourself using body filler a lot, you should try out my other recommendations. Here’s a list of the best car body fillers that I’ve used.
U-Pol StrongHold 7061 High-Density Plastic Filler
U-Pol Stronghold 7061 is a high-density body filler designed for use on most plastic components. It’s ideal for repairing bumpers!
As long as you’re prepping the area with P120 and using a water-based degreaser, it will stick to most plastic found on cars.
PP, EPDM, PP/EPDM, TPO, and ABS are all listed in the technical data sheet as suitable surfaces.
The other thing I love about this product is the fact that it encourages you to repair holes with it.
U-Pol says “Skip the gun, cartridge, and tip and switch to U-POL 7061”. This filler can be used with mesh to fix holes and perform tab repairs.
There aren’t many fillers that do that!
As long as you follow the TDS, use the right amount of hardener, and sand after 30 minutes, this will be the best plastic filler you can get.
It is a little tougher though and you’ll have to start sanding with P120 paper, all the way through to P240.
U-Pol Stronghold is fairly priced, in line with the competition.
A 600ml tin is going to set you back around $50-$70. As long as you’re shaping the plastic before filling, you’ll get a lot of repairs out of 600ml.
Evercoat Poly Glaze
When finishing off repairs, sometimes you just need a thin layer of glaze. Plastic repairs are no different.
If something feels a little off, or if you have pinholes in your repair, just glaze it. This thin, lightweight plastic filler will allow you to fix minor imperfections with ease!
Evercoat Poly Glaze works on SMC, Hard Plastics, Flexible plastics, Composites and Polyester fillers. This more than covers the list of plastics that you’ll face in auto body repair.
As it’s a finishing glaze it only has a 3mm fill depth, but ideally, you’d be heating and reshaping the bumper anyway. This means you won’t need to fill anywhere near 3mm.
Once applied, it should only take 15-20 minutes to dry if you follow the 2% hardener recommendation.
It’s also very easy to rub using P180 grit paper. There’s no need to go any harsher than that.
While I personally can’t recommend this filler enough, it can seem a little pricey.
Available only in an 880ml tub, it could set you back anywhere from $50-$100.
If you’re shaping the plastic well before filling, The 880ml will last for a long time as you’re only applying a thin layer each time.