Thursday, September 4, 2014

Quarter End Cap & Rear Valence Work

Like all detail work, the process of creating subtle custom touches that don’t look like custom touches is extremely time consuming and tedious.  Such is the case when massaging the quarter end cap shapes and fits along with the rear valence corners.

The goal of the last few weeks has been to match the quarter end cap contours to the finished rear quarters to achieve a more blended and smooth look between the parts without changing the “stock” look of the body.  As simple as it sounds, the amount of work that goes into this deceivingly complex area of the car and the time involved in addressing these highly variable surfaces is very high.  As such, the photographs included in this update will not show dramatic differences in these areas, but rest assured, the up close and personal review reveals a significant improvement in look.  In fact, once the whole area is in primer, it will be very difficult to identify the true extent of these modifications, but the overall smoothness and refinement that will result will be a subconscious “home run”.  These are the kind of modifications that keep the knowledgeable admirer coming back to a car, time and again, to try to absorb all of the details they can’t immediately identify.  I love cars like that!
Following up where we last left off, the first detail that must be addressed is to carefully align the trunk lid and end caps to establish the most consistent panel gaps possible.  Once this is set, it’s time to machine aluminum shims that slip over each end cap mounting stud to allow the gaps to be precisely and repeatable duplicated each time the end cap is removed and reinstalled.  Due to the variability in the factory stampings and welded joints, each shim must be custom machined for each location to achieve the tight gap fit required.  Once each shim has been machined, it is indexed to its respective stud so the gaps assemble correctly every time.  From there, the shims are then carefully glued to the base of each stud with epoxy, making sure to avoid getting the epoxy adhesive in the threads.  The end cap is immediately installed on the body in the exact location that achieves the best fit, and then left overnight to dry completely.  With the shims permanently bonded to the end caps, it is easy to achieve the perfect gap time and time again during the fitting and smoothing process.

Next up, the painstaking work of filling an smoothing the end cap contours and lower valence corner transitions to precisely match the quarter panel contours begins.  The work begins from the “bottom up” as it goes and each valence corner is carefully massaged and filled until the transition from the quarter panel into the valence is smooth and the gap consistent on each side.  There really is not “magic” here other than to be very patient and sneak up on the final finish.  This is far more iterative than “collision shop” work and many thin layers of filler will be added and subsequently sanded away until the final shape is achieved.  Get used to this…….it’s part of the game!
Next, the primary attention shifts to the end cap fitting.  The end caps on the 69-70 Mustang are a die cast alloy and allow very little opportunity to reshape them with usual metal finishing techniques.  Fortunately, they are a pretty stable and solid work platform that allows very fine filling work to be done to correct their amazingly lumpy surfaces.

If you stare at the 69-70 Mustang Fastback body as long as I do, you start to pick up on exceedingly subtle idiosyncrasies that ultimately seem……..well……….out of place.  One such area is the way the factory end caps make the tail of the car look ever so slightly “pinched” at the rear such that the quarter-to-end cap fit seems rather ragged.  By spacing the end caps back a mere .050” or so, the contour transition becomes much more pleasing and allows them to be much more cleanly blended using relatively little filler.  After several thin applications of filler and very careful sanding, a final skim of finishing putty is applied to the entire repair to achieve a very smooth surface ready for high-build primer and final finish sanding before epoxy primer/sealer.  One clever trick that I use to keep the gaps tidy during the filling process is to use a razor knife to “cut” the gaps along the clean edge of the quarter and end cap when the filler is toward the end of the gel phase when you can safely shave the high spots with a Surform tool.  This allows me to add filler directly over the gap for ease of contouring.  On narrow gaps like these, you simply swipe filler directly over the gap, but on larger gaps, you must back up the gap with foam masking to prevent the joint from filling up and sagging.  Works for me!
The next phase of work involved preparing the front edge gap on the trunk lid for blending into the lower window panel and to clean up the upper transition quarter end cap edges and rear end cap faces.  This work involves very little filler and makes for a much improved appearance when finished.  You really wouldn’t believe how poor the surfaces on the rear faces of the end caps really are.  In addition, the front trunk lid edge rarely matches the lower window finish panel correctly and now is the time to smooth up this area as well.

So, with several days of work left in these areas, we will soon be moving toward getting the entire tail end of the car in a few coats of polyester high-build primer to allow the final finish work to be done.  This will include flattening the rear quarter flanges to provide a perfect mating surface to the quarter end cap flanges, finishing the outer flanges of the rear valence to present a clean, finished edge to the gaps, finishing up the front trunk lid edge fit, smoothing the rear wheel opening flanges, and scuffing the valence down to bare metal.  Plenty left to do in this area, and plenty left to report in our next update!

Right rear quadrant gaps are carefully aligned before any end cap contouring work can begin.  Gaps are set as close to 3/16" as possible and eah panel mounting location is indexed to allow precise positioning each time the panel or component is removed and reinstalled.

Left quadrant gaps are all set!  This area of the car is rather complex in that there are three (arguably four) critical gaps that must be managed at the same time.  Until each one is set to the exact width on each plane, no other work can be done to the end cap contours.  This is a careful balancing act that takes a lot of time to get right.

The gap at the leading edge of the trunk lid must be equal to the rest of the gaps to keep the refined look intact.  Before it's done, every gap edge in this entire assembly will be carefully massaged for proper fit.

Not to be forgotten, the end gaps on the lower valence receive the same attention as all of the others and the contours of these corners are carefully prepared to allow them to be smoothed with minimal filler.  This area of the 69-70 Mustang body is very often a very disappointing area on may cars due to lack of attention to gap detail.  Not here!

Right side valence gap is metal prepped and ready for a thin skim of filler to get the contour in shape.

The left side valence gap required very minimal work to get into shape.

The right side valence gap was a bit rougher stamping and required a little bit more filler work to get smoothed up.  This was somewhat more complicated based on the fact that the right side quarter stamping was a bit different than the left and required a bit more filler work as a result.

With carefully machined aluminum spacers in place to precisely establish the .050" gap between the rear quarter end cap and the quarter flange, the two components can now be carefully contoured to create a beautiful transition.  This area is another highly variable joint that often looks less than desirable on "stock" 69-70 Mustangs and I am not a fan of seeing this gap filled altogether.

Filler application over a small gap can often be accomplished without masking.  While the filler is in the gel phase, the gap can be cut with  razor knife on the "guide edges" to allow the gap to remain clean and easy to prep.

Left side end cap is now fully blended into the quarter contours and skimmed in glazing putty in preparation for high-build polyester primer application to perfect the surface.

Right side end cap contouring is complete!

With end cap gapping and smoothing complete, the focus can now be shifted to cleaning up the read faces of the end caps and smoothing the front trunk lid to body interface across the gap.
The remainder of gap smoothing on the trunk lid will involve very subtle applications of filler.  here, we have scuffed the primed surfaces on each side of the gap with 80 grit paper to give the filler a solid foundation. 

At the conclusion of this update, here's where we are:  The rear faces of the end caps are skimmed along with the front trunk lid edge area.  Several more hours of work are involved in these areas before we move to the application of high-build polyester primer and hand blocking every surface to the ultimate shape.


  1. Sven, you managed the gaps in 3 dimensions like Troy Trepanier or Chip Foose! I never would have thought that consistent gaps across all the panel joints would make such a huge impact on the look of a car. Absolutely and unequivocally stunning!

    1. Woah Dennis, that is some rarified air there and I am certain I ain't worth that level of compliment! Rest assured I am blushing as a result and most appreciative of such kindness. Around the shop, these have taken on the nickname of the "$3000 trunk gaps" due to the fact I have spent so much "shop" time on them. Nice huh?

  2. Great post and great work Sven! Looking forward to more!

    1. Thank you Grant! More on the way soon!

  3. SVen,
    Is your Mustang built in Dearborn?

  4. .050 !? Now that's what I call an attention to detail. Funny though how such a fine level of precision improves the overall presentation of car in such a big way. Amazing work again, Sven.

    1. Ya gotta have a target! In all seriousness, when you create a gap at the end cap, it becomes so noticeable that you have to keep it very tight and consistent or it starts looking wired. My eye has always balked at 1/16" gaps in this area, so .050" makes for a good target to shoot for to keep it tight enough to look good but not so wide as to look completely out of place. Tricky business! You are probably laughing hysterically right about now...........8^)