Bondo repair cracked off! Help a Noob!

[JAG.]

Hey all, hope everyone is safe and healthy. The headline is that i tried a small bumper repair and did an ok job but now t's cracked again! Here's the story..

So i'm a complete newbie to body work, but my wife's Highlander has a few bumps and scratches i thought would be a good start to practicing and learning the skill for minor body work repairs. i watch a million youtube videos and online guides and i saw it enough that it didn't seem so bad.

There was a small dent/scratch on the rear lower bumper valence that seemed like a good place to start. If i ruined it, it wouldn't be so noticeable since it was pretty low on the vehicle.

I ordered the paint mach in a spray can and bought a bumper repair kit and got to work while the paint was shipped. The instructions were straight forward and i also had youtube videos on while i did it to make sure i didnt wreck the whole thing. To be honest, it went a lot better than i thought it would, considering i'd never done this before and had no idea what i was doing. You can see in the pics overall it wasn't a terrible result.

I sanded it, Bondo'ed it, sanded, primed, sanded again and cleaned it for paint. The paint came and i gave it 4 coats, waiting at least 20mins between coats. I gave it 2 full days before i clear coated it. Gave it 3 coats of clear coat, let it cure, wet sanded it and gave it a final clear coat. Then i reinstalled the valence and it looked alright.

About 2 weeks later i checked in on it and found it had cracked open and chipped off! The paint, the primer, the friggin Bondo CAME OFF!! It's all the way back down to the bare plastic! Around the edges of the hole, you could stick your fingernail in there and chip off the rest of it if you wanted, but i decided to leave it alone and not make it worse.

So what the heck??? Is this normal?? What'd i do wrong guys?? I searched but didn't find any helpful videos about bondo chipping off after painting. Does anyone have an answer for this?? How do i correct it without this happening again?

Thanks in advance!

[Jaymzz]

For plastics there are some really good repair products out there. "bondo" won't stick to plastic very well because it can't get any adhesion to the plastic. At work I use an adhesion promoter and 3M ezsand #05887. It works very good and in the autobody industry a go to for many things and that is why 3M changed the name to multi purpose. There is other products similar like Kent #6000 that will also work the same but you will always need to use an adhesion promoter for any flexible repair. Hope this helps a little.

[JAG.]

Ah well that helps a lot actually! So Bondo doesn't stick to plastic! i guess that settles it. And thanks for the leas on the better materials to use, i'll have to track them down.

I wonder why the heck they'd put Bondo into their bumper repair kit when most bumper covers are made pf plastic these days. Nonsense

[thinkfastroth]

3m also makes a flexible bumper repair 2 part that should bond a little better also some body fillers have adhesion promoter built inform the manufacture.

[Kevin Campbell]

In addition to using the proper materials, a bigger issue may be subsequent impacts. Looks like it’s been hit again after you reinstalled the lower shield.

[dhutton]

In addition to using the proper materials, a bigger issue may be subsequent impacts. Looks like it’s been hit again after you reinstalled the lower shield.

I was thinking the same thing. It cracked because someone backed into something or someone bumped into it. The flex cracked the repair.

Don

[jlcustomz]

Another hit can easily crack a repair , but particularly if not done right.
Plastics, just like metal, should be straightened best as possible for minimum filler. Most bumpers now are thermal plastics, which, mean they can be re manipulated with the correct amount of heat & method.
Another general rule with plastics is if it floats, adhesion promote. plastic with paint still on it doesn't need it, but raw absolutely must have.
Flame treating is another method for prepping oily plastics. Google it, you're not frying the plastic with heat, the flame oxidation changes the surface.
lastly, need a proper filler. I've used evercoat euro soft and 2 part epoxy from Polyvance. Google them also, they have instructions .

[JAG.]

Great info thanks guys. i'm still trying to take this on! will post updates