Advice needed: Cracks forming on Rear Body Panel

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talibraza101

talibraza101

Joined
Jul 16, 2025
Threads
10
Messages
75
Location
MD, USA
Hey guys,

My FJ40 is parked under a tent outdoors. Unfortunately, the evening sun heats up one side of the truck more than the other, and I’ve been starting to notice a few cracks forming on the driver’s side rear tub. Attached are some images:
IMG_3873.webp
IMG_3870.webp


They seem to be getting worse and propagating day-by-day. Any advice on what I can do to prevent this? Will it just keep getting worse due to the truck being parked outdoors in the heat?

I do plan on repainting the rig at some point in the future; any advice on what I can do to fix this when the time comes? Or will the cracks continue to worsen until the whole panel falls off?!

I appreciate any advice 🙏🏽
 
Again… lipsticked from columbia

Bondo
 
Btw no the panel will not fall off … looks like it’s at bottom as well as along the wheel well on the quarters?

If it’s just covering over rust … it’s going to get worse as moisture is getting in the bondo and then drying out in the sun… rust will get worse as well…. OR….

If it’s just cracked bondo because it’s shrinking in the sun (expanding and contracting) it will get worse over time as moisture will get in

What does the underside of the panels look like?
 
It's not going to get any better. B4 painting, the bondo should be removed and taken down to metal to see how deep it goes. This is the problem with South and Central American 40's. Short cuts galore and then lipstick.
 
Thank you both for your responses, unfortunately I’ve been learning the hard way about these South American cruisers 😬.

JohnnyC said:
Btw no the panel will not fall off … looks like it’s at bottom as well as along the wheel well on the quarters?

If it’s just covering over rust … it’s going to get worse as moisture is getting in the bondo and then drying out in the sun… rust will get worse as well…. OR….

If it’s just cracked bondo because it’s shrinking in the sun (expanding and contracting) it will get worse over time as moisture will get in

What does the underside of the panels look like?
Click to expand...

This definitely feels like bondo cracking through thermal expansion. The underside looks like this all over the rig, so hard to tell whats really going on behind the cracks:
IMG_3874.webp
IMG_3878.webp


Thoughts?
 
Looks like a poor weld in the spot… is that where it’s popping?
 
JohnnyC said:
Looks like a poor weld in the spot… is that where it’s popping?
Click to expand...
Yep, thats where the major crack is forming. Right at that seam.
IMG_3847.webp


Thanks!
 
That's a typical location for rust along with the rear sill. It appears to be where the top of inner fender meets the 1/4 panel. The pic of the inside of the wheel opening looks like there was minimal repair in an attempt to fill the holes, then they covered it with bondo. Applying bondo or paint over rust is a recipe for disaster. It actually promotes rust.
 
Yeah it definitely is unfortunate; thankfully all cosmetic though. What do you all think the best approach would be to fix this? Assuming the base metal is still in half decent shape underneath, would it involve just grounding out the old, brittle bondo and applying a fresh filler application? Or am I probably looking at something more serious that needs proper repair first? Thanks for all your help guys.
 
Rust pushing filler outward. Use scraper/flathead to pop off the cracked filler as much as will easily depart, then I would use a flapper disc to chase the sub rust until you get good material, feather the edges, prime with heavy bodied primer until you can repair/repaint as you want. FWIW to remove that textured undercoating is a task, DON'T get in your eyes. Alot of flapper wheels as they fill up fast, a chemical scrape prior to flapper or 3m wheel helps.
 
Thank you all
 
So turns out my magnet is staying true all over the truck, even on the cracks i mentioned. So I guess it isn’t bondo lurking behind these cracks after all? Im puzzled…
 
Bondo doesn't stop a magnet from sticking unless it's very thick. A magnet will stick less and less the thicker bondo is, but it'll still stick. From your cracks, my guess is that you've mostly got less-than 1/16" of bondo on there, but they didn't prepare the metal before applying it.

The good news is, sitting outside, it won't be long until that bondo pops off. So you'll be able to see shape of the metal underneath soon enough.

My guess is it'll need some repair work, then you can redo it properly and it'll last a good, long time.
 
Ahhhh okay gotcha, thanks for helping me understand better 👍🏽

Just dug up an old pic from the truck’s “restoration” in Colombia. I can definitely see some areas where bondo was used close to the spot on the quarter panel that is currently cracking. (Left corner of image)
IMG_3850.webp
 
Bondo .
They probably just skim coated it, which is an acceptable process....but used bondo instead of a glazing puddy.
 
That prep is dogshyt, I'll bet most of that never got scuffed. It's going to be a problem everywhere on that body, and I'll bet there are already rust pockets forming. It's an unfortunate thing, but almost every single rig I've worked on that is from SA has always been that way.
 
It looks like that rear sill has been poorly replaced with a lot of holes; that's going to be a problem area in the future. Pinholes in sheet metal welds are killers.
 
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