Monday, November 21, 2011

Tail Light Panel and Trunk Floor – Part 3: Wheel Opening Rear Corner Repairs

As I mentioned in my last posting, the left rear corner of the wheel opening was found to be peppered with tiny rust holes, discovered after sand blasting the areas in preparation for the trunk floor repairs.  This changed the pace of the trunk floor repair a bit in that I had to make this repair before I could go any further in repairing the trunk floor, since the trunk floor drop-off must be welded to the inner wheel opening flanges.

I spent quite a bit of time studying the overall scope of this repair along with the repair panels available for the job as well.  Fortunately, all of the sheet metal I would require was indeed available.  However, I would need to buy a complete outer wheel house and quarter patch panel to get all of the stamped parts necessary to do this work the right way.  Another trip to NPD to emasculate my wallet………

One of the “tricks” to effective rust repair is making sure your repair will be made far enough beyond the rust damage to get you into good solid metal on every edge.  It is very easy to assume a few small pinholes of rust on the outer surface equate to a small repair, but as you will see below, this is generally quite different from actual fact.  Experience has taught me that a general rule-of-thumb is the “spread” ratio of rust is roughly 3:1.  In other words, for every ¼” rust hole you can see at the surface, the actual damage (e.g., what you can’t see) is actually equivalent to a hole at least ¾” in diameter and the required repair will have to accommodate this larger specification to get into good solid metal.

In consideration of the above, it is still prudent to start small and expand the scope of the repair by small nibbles until all of the damage can be removed without sacrificing a bunch of good metal.  Once I had a good idea of how extensive the repair would need to be, I marked the general area with my white paint pen and transferred this same marking to my patch panels for future reference.  Next, I grabbed my handy dandy “screaming-metal-eating-wheel-of-death” and carefully cut the rusted corner of the outer sheet metal about ¼” inside of my painted line.  Then, I marked and drilled the few spot welds along the front and lower flange areas and off the piece came.  In keeping with my 3:1 rust spread ratio above, the extent of rust damage was indeed much more significant than the outer skin would show.

With the outer skin removed, I moved to the damaged section of the outer wheel house that was directly behind the skin.  This was where the real extent of the rust would become painfully obvious.  With careful evaluation, I was able to establish how far up the wheel house the patch would need to extend and then transferred this measurement to my new wheel house.  Then, I cut the wheel house patch on my band saw and cleaned up the edges on the belt sander and it was ready to go.

This brings me to another tried-and-true, but rarely mentioned rule about fitting patch panels:  ALWAYS make the patch before cutting the parent metal.  This ensures the patch will be properly sized and shaped to the task at hand before you ever cut the original damaged metal away.  With a patch that is formed perfectly, you can simply clamp it securely in place and trace the exact cut line you need to follow using the patch as the template.  If you configure your scribe to incorporate the required kerf offset, you can cut just under the line and use a 2” disc grinder to sand the base metal right to the scribed line.  This makes fitting patches for butt welding far easier than you might expect.

Once I had my wheel house patch trimmed and fit to the car, I used my trusty panel clamps to secure the patch in place at exactly the right gap for welding.  In a matter of a few short minutes, I had the patch butt-welded in place and the welds ground smooth.  This was now an excellent base to work from as I moved on to making the outer corner patch and welding it into place.

Since I had already marked my patch panel with the same basic shape as the damaged area I cut away, I could easily cut a patch that was a bit larger than the marks to allow plenty of extra material for custom fitting if necessary.  As it turns out, the patch required only a little bit of trimming and it was ready to go.  As with the wheel house patch, I let the patch determine where the cut lines needed to be on the original quarter panel and then I scribed my cut lines and made the final slight trims necessary for a perfect fit.  Again, using my panel clamps for secure location, I tacked the patch into place about every ½”.  I like to use a lot of tacks on the first fit as they not only act as a method of securing the panel, but also as very effective heat sinks in the process.  This is a big help in keeping warping to a minimum, particularly when using a MIG welder for such work.

Once the panel was fully tacked in, I carefully ground the tacks flush with the parent metal to have one last look at the fit before fully welding it in.  By grinding the tacks smooth at this point, it allows me to feel the fit of the patch and make sure the contours and edges match exactly as they should.  This helps keep the amount of filler required to bring the surfaces up to a minimum and give you one last chance to make corrections before welding it in permanently.  And as luck would have it, the fit was excellent and I set off welding the patch in fully, using long blasts of compressed air to chill the short, ½” stitches before moving around the panel randomly to complete the weld with minimal distortion.

After all the welds were complete, I ground the seams smooth and verified the welds had good penetration to the inside of the panels.  With everything in order, I was finally able to trim the inside flanges to replicate the factory profile and the repair was complete.
Using a paint pen, I marked the general size and shape of the required patch on the repair area.  This line is significantly outside the immediate damaged area to ensure the repair is made into good sheet metal.

I transferred my basic patch dimensions to my patch panel material for reference before cutting.

Removal of the damaged outer skin area starts with drilling out the spot welds along the front and bottom flanges.  Then the skin is cut to the inside of the paint lines with a cutoff wheel.

You can see the extensive rust damage under the skin and plenty of light passes through the pinholes in the inner wheel house.  But when the lights go out.....

.....the true extent of the damage becomes obvious.  OUCH!

Here is the back side of the outer skin section that was cut away.  What a mess!

In the same fashion as I marked the outer skin, the wheel house corner was marked and cut.

ALWAYS let the patch panel determine where the cuts are made in the parent metal.  Here the patch has been formed and metal worked until the fit is perfect.

Even this notch was reproduced in the patch and it's location verified before any cutting was done.

Here, I have scribed the cut lines and marked some indexing lines using my paint pen.  Then, I used my cutoff tool to cut away the damaged metal and the, using my 2" grinder, I trimmed the panel to match the scribed cut line exactly.

With the fit exactly as I wanted, I clamped the patch into place with my panel clamps and was ready to weld.

Here, all the welds are completed and they have been ground flush.

Here's a nice shot showing the outer patch panel tacked into place.

With the tack welds ground smooth, I can check one last time that the patch fits correctly before final welding.

The patch is fully welded using short stitch welds placed randomly around the patch and cooled with compressed air between welds to keep warping to a minimum.

With the welds ground smooth the patch will require very little filler to get everything smooth.

Weld penetration on both patches is excellent.  And best of all:  NO MORE RUST!

The repair was finished by trimming the flanges to match the factory shape.  Done!


  1. Excellent walk through on how to patch a panel and very nice work! Your technique on keeping warpage to a minimum is awesome but it should also be mentioned that grinding can also warp metal if not done carefully. For example, if you concentrate the grinder in one spot, say working the weld flat from one end instead of lots of complete passes from end to end, the panel will warp just as surely as if you had overheated the joint with a welder. Ask me how I learned that. :-)

  2. Well Sven, after seeing how you handled the butt weld of the patch panels and seeing the gaps you had, you gave me hope I'll be able to get the floor in my Mustang in and hopefully looking as good as your work. Love the detail in both words and pix. Thanks for sharing...again.

  3. Alex, you couldn't be more right on that detail. Grinding heat can absolutely torpedo a patch panel project in a hurry and I should have mentioned that for sure.

  4. Dennis,

    Thanks for your compliments. I was just over looking at your floor pan install and I have some thoughts/tricks that might help. I'll drop a comment in hopes it will help.

  5. “screaming-metal-eating-wheel-of-death”

    Boy is that ever an apt description. As useful as that thing is, it still scares me to run it. I think, each time of use is another opportunity to get to know the emergency room staff better :/