The most prominent rust spots are located at the lowest section of the front driver and passenger door panels. This is a design flaw. The engineers had to press multiple layers of sheet metal together at the very bottom of the door. The compressed metal seam is covered with a simple plastic trim piece, sans rust-proofing sealant. The trim would rot and trap the moisture between itself and the metal seam, causing rust to build up and expand.
The good news is that the metal is pretty thick, so you may have a chance at saving the door even if the paint is already bubbling, but the repair requires professional rust elimination and proofing service. In a lot of cases, the amount of work outweighs the cost of a new door.
Also, check the water escape hatch located on the interior side of the door. Even if there is no visible rust, it's always a good idea to check for active blockage.
Check all of the jacking points for rust and structural integrity. The jacking points on these cars usually start to rot at 100,000+ miles, which could lead to a catastrophic failure when your car goes up on a jack or lift.
My recommendation is always to use frame rails when jacking the car up and never rely on the jacking points. In my opinion, all of the jacking points in these cars are compromised and should be cut-out and replaced with welded high-grade metal.
The front floor pans have a water evacuator hatch that tends to degrade and rot over time. Jack up the car and inspect the metal around these hatches. Also, take a good look at the space where the floor pan joins the side fender.
Check the mount points for the bump stop. The protective surface paint begins to strip after years of opening and closing of the trunk. If the spot is rusted through and the integrity is compromised, my recommendation is to source a new trunk. Otherwise, you can try stripping the surface rust and coating the area with a quality rust converter, such as POR-15.
Check the mounts for the license plate bulbs. You can temporarily repair and patch the spot with Bondo, but a proper repair will be hard to pull off for an amatuer due to difficult access.
Check under the carpet, behind the plastic guard that sits on top of the lock hanger.
The rust tends to form around the gas door and the supporting frame. Carefully inspect the door, hinges, and the metal behind the filler neck gasket.
The sunroof panel is prone to rusting due to scratches caused by normal wear and tear. Keep a very close eye on them. Look inside the sunroof panel for signs of rust, and check behind the surrounding rubber seals.
In most cases (especially in lowered cars), the fenders will be bend and cracked around the wheel well. That, combined with years of rock chips and cycles of moisture build-up, makes your fenders a very likely target for rust.
Check the areas around the wheel well and behind the side skirts.
The tail light seals tend to fail and leak water to the rear corners of the trunk, which causes the trunk to rust from inside out. Thankfully this is a somewhat easy fix as the area is not as restrictive.
Take out the taillights and inspect the surrounding metal from inside and outside the trunk.
My notes on removing and repairing rust
The most annoying thing about owning an older car is rust. You might think that your car is rust free, but the unfortunate truth is, it has rust.
My amateur (so-called) restoration work was covered by Jalopnik a while back. That was a great learning experience, but I don't recommend undertaking such a complicated project by yourself. It's a lot better to redirect your energy into a second job and use the extra money to pay a professional that has the right equipment, space, and a looot of patience. My car never felt polished and I ended up paying for a professional restoration.
Below are some of my process notes that could be used for smaller rust control jobs. It's a good idea to set a 12-month inspection follow-up in your calendar for all the rust repairs.
- Do not use rust converter or stripper as an easy way out. These chemicals should be used in conjunction with other tools
- Blast the area down to the bare metal with a media blaster. You can also use a metal grinder mounted on a Dremel or drill
- Use a wire brush coated in rust converter and aggressively massage the converter in. Let it soak per instructions
- Clean the area again
- Get a commercial grade rust stripper and follow the instructions. Use the wire brush on the affected area to speed up the process
- Repeat the steps until the metal rust and particle free
- If the rust ate through metal, you might be able to save it with Bondo if you are on a budget or rush, but a welded metal patch is the way to go if you want to do the job once
- Sand the surface and surrounding areas down with 160-300 grit sandpaper
- Carefully clean the surface with pre-pain cleaning wipes
- Mix, apply, and spread Bondo – filling and overflowing the bare metal along with the surrounding area you are going to patch
- Smooth it as much as possible. The patch should be raised and cover the non-sanded areas. You want to use as little Bondo as humanly possible
- Follow with 600-800 grit sandpaper to smooth and blend the patch with the rest of the repair surface. You have to use something like Durablocks to guarantee even blend
- Apply quality sealable primer
- Sand the primer down with a 1000 grit paper. Again, you want to use Durablocks or similar for this step
- You can start the painting process (or undercoating) once the primer fully cures and you are satisfied with the result
Messing around with Bondo patching
If you don't have access to a welder and can't weld yourself, you can try repairing small holes in the body panels with Bondo. The process mirrors your typical wall patch repair job that you can do at home. You may want to do a couple of test runs on drywall to get the hang of properly blending the repair first. The mistakes should be a lot cheaper.
Disclaimer that patching with Bondo is not recommended, and the application heavily depends on what you are attempting to patch. I think a sunroof panel might last a couple of years, while a repaired floor pan will collapse as soon as you put pressure on it. So yeah, common sense.
Use a small stainless steel plate and cut it to the exact size of the hole you are attempting to patch. Work the Bondo between the plate and the metal you are plugging from both sides of the patch. You will need to use several layers of Bondo.
The metal plate will serve as an additional bonding surface and provide greater structural integrity to your repair.