Advice needed: Cracks forming on Rear Body Panel

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Thanks for the input everyone. After using a weaker magnet I can definitely tell there is a layer of bondo behind those cracks that are forming. Definitely a bummer knowing there may be rust pockets lying under the paint. I do like @Chamba ’s idea on waiting until the bondo pops off and then having a clearer picture on where to emphasize repairs when its time for bodywork.
 
You really need to open it now. Waiting will allow moisture to be trapped therefore accelerating rust, that's already there.
 
My understanding is cracks in bondo could have different causation with two of the most common being inadequate surface preparation, and bondo that was applied too thickly. At this point you really don’t know what caused the cracks, or what lies beneath….

I realize there is a strong temptation to investigate, but here’s the thing: Paint and body work is not cheap, and that's especially true when it’s done right.

This rabbit hole could be long and expensive. How deep are your pockets (and I ask that not to be rude, but rather as someone whose projects NEVER come in on budget)?

Are you really sure you want to open this can of worms?
 
If anybody is a visual learner, the PO of my truck passed away before he could finish the restoration, and my guess is he did exactly what they did with your truck.

The tub had rust out all of the way through the metal. There is bondo oozing out of the back under these side panels on mine (sanded and primed on the left side, just bondo’s and left on the right side, inside just has the hardened tried bondo like the keys of lathe and plaster wall).

Where you have the bondo on your inner left side, I bet that was a hole all the way through.
IMG_1510.webp


IMG_1531.webp
 
This thread brings back horrible memories.

Rust.
Bondo.
Maryland.
More rust.......
 
Hi guys, thanks so much for your responses - definitely some scary stuff! A few things I wanted to mention:

- I absolutely want to do all bodywork, repairs, and painting myself. I do have a good amount of basic equipment already (welder, paint equipment, etc.) that I've been passed down by my father, though I'm not exactly the most skilled with any of them 😅.

- The driver's side 1/4 panel that is cracking is about the only area that I see visible cracking on the truck at the moment, and is where my magnet is extremely weak. As @nabbasi pointed out, I definitely think there is bondo straight through the body here.

- I am currently finishing up a drum brake overhaul (single to dual MC conversion). Its been months since I've even driven the truck so I definitely want to get it back on the road again before taking on this bodywork overhaul. That being said, the cracks are worsening day by day and @brian mentions that I should open this thing up ASAP. With the truck being stored outside at the moment, it definitely is exposed to harsh sunlight and dampness.

Question: How soon should I realistically be ripping this thing open? Do you think its a bad idea to wait some time until the cracks worsen and I get a clearer picture of the situation underneath as others advised? I have a feeling this area is suffering solely from thick bondo application and not horrific rust just yet. I really appreciate yall's advice - been dying to drive this thing again.
 
Last edited:
I dont see the urgency in doing the body work immediately. Fix your brakes and enjoy the summer. If bondo falls out, so be it. I generally do a lot of my work later in the year and thru the winter and early spring. I think once you tear into it you'll certainly find rust. You'll be cutting it out and replacing with new metal and treating it. Fortunately most of the panels are flat. It will take time. It's going to get ugly b4 it gets better. Watch repair videos and check out the paint and body section here. In this section there have been a lot of builds where typical repairs have been done on 40's also. Like 1/4 panel and rear sill fixes and repairs. I usually prefer to do mechanical repairs b4 attempting to do body work, which I struggle with.
 
If it's just peeling in one spot but not yet rusting, the faster you can pull off the peeled mud and reseal the metal underneath, the better. Mud always attracts and holds moisture, which is why you are usually susposed to do a seal coat of primer before any application of mud. If it's rusted out, it's just going to keep affecting a larger area. It doesn't look like it's bubbling super badly to me, just cracking. I've had to fix so many rust pockets caused by improper mud and bodywork techniques, and they almost always stem from improper prep uppon application, or a pinhole from a patch panel with moisture getting in from behind.
 
My few cents: 1)Having busted out some bondo over rust you will likely be grinding out bondo at least 6” in all directions and likely chasing it!. 2) based on that location it is likely to be prolific.. , at minimum would expect much of both rear quarter panels and the rear sill.. 3) you might try one of there tools: I have not.. but I have seen both the expensive (YouTube pro shop) and the 80-100 ones work, probably some nuance /learning curve, your not only picking up bondo thickness but diminishing steel, but you should have places on hood or cowl as an anticipated baseline) This will help you plan and budget for a big job! And good for you!
IMG_0155.webp
 
Thank you all for your feedback. I will update this thread when I get to sanding these cracks down!
 
I can already smell the metal and bondo dust from the flapper disc now.
 
talibraza101 said:
Thank you all for your feedback. I will update this thread when I get to sanding these cracks down!
Click to expand...
A "surface conditioning tool" from eastwood or harbor freight will make quick work of the paint and filler. It will take you straight to metal with the right drum.

On the other ends of the cost, control, and comfort spectrums is the Mirka Deros line of sanders. I sanded an entire car down for a full respray in 4 hours, without any noticeable dust in the air, and no vibration fatigue. They are spendy, but worth it if you could use a quality sander.
 
I wouldn't touch it until it was time for a full body strip and respray.

Your problems likely aren't confined to that area, If somebody did poor prep and just put filler over rust, that is just the first place you were seeing it.
I wouldn't trust anything on that body, and if you want it done right the whole thing should come down to bare metal and fix all the rust or you will be chasing bubbles for the life of the truck
 
I really don't think there's any hurry. You're going to be replacing much of this metal anyway, so if a little bit more rusts, it's just a little bit more you're going to have to replace. In the grand scheme of things, it's minimal work relatively speaking. This is a great vehicle to learn your body welving skills on because everything is flat and relatively thick metal.

I agree with the above: keep enjoying it, get it mechanically where you want it and just drive it until you're ready to do the bodywork, and then do it all it once. If some of the bondo falls off, so be it. Get some blue paint that matches as close as you can and paint that spot.
 
Last edited:
Get the rusty bits absolutely dry, use a heater carefully then get fishoil in to all the cracks, even if you've got to pick a chunk of bondo off to make it easier. This will wick everywhere and hold things for a while. Bought a John Deere 4440 that had a big smack in the nose, which doubles as the diesel tank, po had repaired it with bondo which was cracking and lifting. Fishoiled it over 10 years ago.....
Then go and enjoy your truck
 
@talibraza101, do you have anymore pics of the 40 prior to the body work and paint?
 
Unfortunately thats the only one I’ve got. Trust me I asked 🤣. Thank you all for the amazing advice and feedback!
pb4ugo said:
@talibraza101, do you have anymore pics of the 40 prior to the body work and paint?
Click to expand...
 
IF you're keen on doing all the bodywork yourself you've got the perfect project. There will be big issues, that entire bottom panel has probably got pinholes over all of it and you've found the rot in the worst panel. Honestly, go with the fishoil while you sort the mechanicals, then have some fun and once you hook into the body do it in stages - you've got lots of good metal hiding in there, too. There is almost no shape in a Cruiser body, everything is a flat panel except parts of the fenders, hoods and the roof corners on a 45, I taught myself everything I know about migging panel steel on one of these, Nike School of Everything, just do it. I've posted a couple of threads - don't know how to link.
If you decide to hook into it, cut the rot out, do the repair, paint it well enough to hold it until you've done the whole tub then get it sandblasted and epoxied - then repair the bits you didn't know about, then paint.
 
Last edited:
You must log in or register to reply here.

Share this page

Back
Top Bottom