Wednesday, September 16, 2009

Beater How-To: Unibody Frame Rust Repair

Beater How-To brings you how-to guides for various repairs that eventually every beater will need. This tutorial details how to undo the damage mercilessly and intentionally done to your vehicle by the Mid-West: Unibody Rust Repair.

While making the weekly beer and food run in the Simca, my girlfriend noted that she was having to put a bit of shoulder action into the passenger door to get it open when only a week ago, all three of the passenger doors willingly popped open and slammed shut when asked. Something was clearly amiss.

I took the car out to the garage and put it up on the lift for the first time - a scary experience for each vehicle. Upon first glance, it didn't look terrible. The steady oil drip out of the motor and transmission acted as a pickling agent for most of the components in the center of the car, leaving most items very well preserved. Then I looked towards the rockers - and then I poked it lightly with my finger and found a sizable emergency entrance to the cockpit that spanned the entire wheelbase and extended to the driver's side frame horn. Maybe that's why the doors were sagging...

This series of how-to guides will detail how to repair said damage, starting with most obvious place: the driver's side frame horn. I have as much experience with body work as I do with writing how-to guides, so keep that nugget of information in mind before you follow this guide to the tee. This is what I did to keep my car from breaking in half, not to prepare for the Monterey Historics.

Tools and Materials Used
Angle grinder
-Braided wire cup wheel
-4" Cutting disk
1/4" 90 deg. air cutoff wheel
-3" cutting disk
-2" cup-style surface prep kit
Port-a-Band (Not necessary but makes life much easier)
MIG welder
Body hammer set
4 Cowboy clamps
Spot weld driller
Pry Bar
16 ga. steel (1 sq. ft.)
3M Undercoating

Step 1: Gain Access to the Area
The car was raised up on the lift by ensuring that the lift pucks were not touching future areas of required repair. The front left wheel was removed to provide better access to the frame.

Step 2: Brace It
In order to make sure that the front didn't fall off, a brace was required. Being that I was only going to work on one side at a time, I figured that I could get away with bracing one side at a time. The jack post was made by cutting a steel tube to length and affixing two rubber footed screw legs to the rod, allowing for the front of the car to be supported by this turnbuckle-like device.

Step 3: Remember The Way Things Were
Before going tornado-in-the trailer-park with the braided wire cup wheel and angle grinder, take a minute to document things. With the body hammer, tap the rusted and damaged metal into
something that somewhat resembles the original shape. At this point, put the rest of the PBR in the fridge and cut up the box to make some templates of the sheet metal and rust matrix before it vanishes. Don't forget to take pictures and measurements if necessary. Clean up as much of the rust chunks, undercoating, and other potential projectiles or fire fuel. I found these acorns!

Step 4: Rust Massacre
Go to town with the cup wheel and destroy all rust. This will give clear view of what you have to work with in terms of weldable metal, provided you didn't forget the safety glasses. In this case some other parts of the inner fender needed to be drilled and removed to access all of the rust.

Step 5: Mark The Bad Areas and Disassemble
With the rust gone, mark the bad areas that need to be cut out and replaced. In this case it was a large dual walled section of the outboard side of the frame, as well as two layers of metal making up the bottom of the frame rail. Trim the templates accordingly and set them somewhere where they can't catch on fire. With the hammer and punch, divot each of the spot welds attaching the layers of metal to each other and drill them out with either a spot weld driller or a Roto-Broach. Tap the metal around them to ensure that the layers are completely separated.

Step 6: Make Sparks Fly
This is the point of no return. Grab the air grinder with the thinnest of the cutting wheels and start tracing your marks with it. The outer frame layer needed to be completely removed in a roughly 10"x4" section. As you can see, this only uncovered the worst of the damage to the inner portion of the frame. Using a sharp ended body hammer, the entire area was lightly tapped to determine where the solid metal started. It's really easy to be too gentle here. Remember, if your hammer can go though it, or if the metal is easily dented, replace the section. It took a lot of effort to avoid the mental gymnastics of trying to convince myself it was all ok. As you can see from the bottom of the picture, the rib at the bottom of the frame was removed from the box section. It will be reconstructed later.
As you can see, very little of the remaining metal was actually solid, so I cut the opening to within 1/4" of the outer opening, which was known to be solid. To my great relief, the inside of the frame rail (which I could now nearly fit my head into) was completely solid. Finally.

Step 7: Make New Inner Section and Weld It In
A template was now made by placing a piece of beer-box over the opening and tapping lightly with a hammer. In most cases, the template will cut itself out this way. The new metal was cut with a Porta-Ban clamped in a vice. When making repair sections that will be butt welded like this, try to not leave gaps more thicker than welding wire. Also, lightly chamfer the edges of the metal so the weld bead will lay flatter. Start with the voltage set fairly low to avoid burning through the metal and experiment with your settings by first tacking the perimeter of the work, as shown to the left. With the piece tacked in place, dress the metal around the weld by lightly tapping the edges to make a level perimeter. Next, grind the excess weld so the outer section will lay flat. As the weld bead thickness increases, so will the likelyhood of setting your hair ablaze while grinding it off. I had quite a few close calls.

Step 8: Make The New Outer Section
Go find the initial cardboard templates that were made for the outside section and make sure they still fit. Use the same methods as in Step 8 to recreate the panel. This panel had one difficult contour where the panel bend upwards to support the floorboard. This was replicated by bending a smaller piece 90 degrees and butt welding it in at an angle and finish grinding. This is done after the panel is otherwise done to ensure that the piece is welded in in the correct orientation to the floor. Took a couple tries to get this one right...

Step 9: How to Replicate a Spot Weld
I don't have a spot welder, and I don't really think that I could've spot welded this particular section being that it was not possible to gain access to the other side of the frame. Therefore, about a dozen 1/4" holes were drilled in the outer panel starting about 1" from the edge of the panel. Using a countersink, or a slightly larger drill bit, a chamfer was put on each of the holes. Using some large cowboy clamps, I fixed the out panel in place and dressed the edges with a round body hammer. The panel was then tacked in place.

With the other two clamps, force was applied locally to the area on the opposing sides of one of the holes so that the inner and outer repair sections were in contact. Using a low feed rate and higher heat, the panels were welded together by placing the MIG gun in the center of the hole and holding the trigger until the chamfer was filled. Take my word for it and start in the areas you'll be less likely to see, gaining skill as you move to the more visible areas. With any luck, it'll actually look like a dimpled factory spot weld! You can always fill them level and grind off any excess if you don't like the dimpled look.

Step 10: The Lower Section
Using basically the same methods listed above, along with some bending, the lower sections were constructed and welded in. Instead of bending the bottom section 90 degrees to recreate the ribbon at the bottom of the frame rail, two pieces were made separately: a ribbon section and the bottom section. This allowed for far less bending, as well as for the corner joining the ribbon to the bottom section to be welded to the outer frame section. See what I mean in the picture down below. The panels were then welded in and ground ground level and to match the other side of the ribbon section. The ribbons were then tacked together. Oh yeah, and when you are welding, remember to cover up the fuel lines. Especially if you have a Simca and they are effing plastic.

Step 11: Clean Up and Undercoat
Make sure the surface is clean, ground, sound, and level. Shoot it with undercoating and the next sucker will never know.

In Conclusion, my goal here is to use a worthless but rare classic to motivate me to do body work. I'd like to say that even if your car needs more reconstruction than the Post-Bellum South, it would be possible given enough welding wire, grinding wheels, and patience...but we'll see. Check back for the Sequel: Floor Board Reconstruction!


  1. Hi

    Really enjoyed this tutorial. Thanks you. Did you ever get around to creating the sequel: Floor Board Reconstruction! ? if you did i can't find it. I have a 1986 Nissan 300zx with rust issues. :-(

  2. nice blog has some good pointers and if u feel a bit over whelmed before doing something like this taking a look at this makes it feel more of a reality and acheivable. reworking a 97 ford escort same issue basically came from up north lots of salt and snow.

  3. If you think this is bad, you should've seen the underbody of my old '89 Beretta... Let's just say getting some new replica rims was at the very bottom of the list of things that needed done to it.

  4. I have to do this with my 85 4wd Subaru that I refuse to get rid of. Instead of MIG, I'm going to be low amp air cooled TIG welding everything back together and filling spaces with rust proventitive foam.

  5. I have to do this with my 85 4wd Subaru that I refuse to get rid of. Instead of MIG, I'm going to be low amp air cooled TIG welding everything back together and filling spaces with rust proventitive foam.

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