Gapping & Smoothing the Trunk Lid

Lots of folks out there have heard the term “gapping” body panels to fit, but few have actually seen the techniques used to achieve these wonderfully “tight” panel gaps up close.  Also, like anything, there is more than one way to skin a cat, as they say, and perhaps the biggest differences in achieving tighter panel gaps is in the welding techniques used.  As part of this update, I will show the methods that I use and provide the reasoning behind the techniques in hopes that it may inspire others to tackle this type of work in their own projects.  Here goes!

Previously, I discovered that my new trunk lid fit was actually pretty nice along the sides and bottom rear edges.  However, as is common on most classic fastback Mustangs, the front trunk lid edge was a mile wide and looked pretty disproportionate to the rest of the gaps around the lid.  Naturally, that would not be acceptable and I would need to remedy this by adding about 1/8” of material to the leading edge of the trunk lid to tighten up the forward gap to match the rest.

Before we get into the “meat-n-potatoes of how this work is done, I will spend a bit of time describing the two common methods of attaching gap edge material to a body panel:  Welding and brazing.  Welding is a popular choice because it is relatively easy, and can be accomplished with either a conventional MIG welder or (preferably) a TIG welder. Either method will work, however MIG welding tends to be a bit messier due to the rather heavy filler metal deposit and the larger amount of heat required to complete the weld.  TIG welding is far more elegant and results in a much smaller bead and far less heat required, thereby minimizing the panel warpage and finish grinding required to get the surfaces smooth and ready for finish work.  However, with either method, the metal will still require substantial heat to reach the melting point and that is something to be avoided when making this type of cosmetic enhancement.

My personal choice for doing this kind of panel work is TIG-brazing.  For some reason, you don’t see a lot of this anymore around body work, but in many applications, TIG-brazing is far superior to welding for a number of reasons.  First, the brazing process requires quite a bit less heat than welding and minimizes the risk of warping the panels.  Secondly, and unlike conventional torch brazing, there is no flux required since the argon shielding gas does not allow the weld area to oxidize, provided the bare metal is cleaned and prepared properly.  This eliminates the concern for contamination of the finish due to flux leeching years down the road.  Also, the silicon bronze brazing material “wets” far better than any weld metal and virtually eliminates any porosity or gaps in the weld area that can form rust later on.  And last but not least, a TIG-brazed joint remains much more flexible without risk of cracking or breaking, which is a huge advantage on old unibody cars like this where seams around the rear window and lower seams at the lower front edges of the rear quarters are often welded up for cosmetic and stiffening purposes.  The obvious “drawback” to TIG-brazing technique is that a TIG welding machine is required and this often is beyond the budget and skill set of many home auto restoration enthusiasts.

The gapping process starts by tacking a single piece of 1/8” welding rod (mild steel rod works equally well) along the entire forward edge of the trunk lid.  Using a single, continuous piece ensures a clean edge that will require very little finish work to achieve a perfect gap.  I find a tack about every 2 inches works well and allows the fit to the edge to be very tight with little more than a light tap with a body hammer on occasion.  For this work, I prefer to use a CK9 flex-head torch with a #7 gas lens and pushing about 35 amps through on average and feeding the puddle with .062” silicon bronze wire. 

I also use a couple of trick little accessories that make this kind of work much easier and more reliable as well.  The first is a little trick I have used for years that helps ensure a good ground on oddly shaped pieces of metal.  Essentially, it is a foot-long pieces of welding cable that has been stripped to bare copper and then folded over on itself and twisted together to provide a copper “pad” that I put under my ground clap to provide a much more solid ground to the panel without denting or scarring the surface.  I never weld on anything without this little widget anymore as it works that good!

The second nifty device is a nice accessory called a “Tig Finger Heat Shield” and is made by a phenomenal welder and teacher named Jody Collier.  This is a small insulating sleeve that fits over your pinky or ring finger of your torch hand and allows you to brace directly on the weld bead without burning the shit out of yourself in the process.  This just might be the best $12 you will ever spend on anything welding related.  You can find the Tig Finger and many other excellent welding wares here:

TheWeldmonger Store

I start my tacks along the bottom edge of the panel and once the entire edge is tacked, I test fit the panel to see how things are shaping up.  I mark any areas that require tweaking to fit and make the necessary adjustments before heading back to the welding table for tacking on the top side and finish welding.  The final brazing is done in the usual “skip-weld” method, essentially welding between tack welds, and cooling each stitch with compressed air before moving on to the next.

Once the top and bottom edges are fully welded, I grind each bead smooth with the air grinder.  Since the silicon bronze material is softer than the steel parent material, it grinds very quickly and cleanly with very little heat introduced into the panel.  Once the rough grinding is done, I finish the edge with 80-grit paper on my DA sander and check the fit of the panel again to make sure nothing moved too drastically.

At this point I discovered some very subtle waves had been created in the panel which I didn’t expect.  After some careful research, I discovered that the panel skin on the new trunk lid was not tacked to the structural panel like the original trunk lids were.  As such, as I welded the edge, the skin began to float on the edge and tweak the flat surfaces a bit in the process.  Luckily this wasn’t too bad and a little bit of torch-shrinking and a few, very thin applications of filler was all that was needed to get the panel straight again and ready for DP40LF epoxy primer to seal things up.  More on that in the next update!

 

Correcting the forward trunk lid edge gap starts by tacking a single piece of 1/8" steel rod along the edge.  Note the extra material left hanging off the end of the panel in this shot.  This will be cut off and metal finished after the entire rod is tacked securely in place.

 

Tacks are spaced at about 2 inch intervals along the length of the edge.

I prefer TIG-brazing using silicon bronze alloy rod and my CK9 flex head TIG torch with a #7 gas lens.  This makes for a very controllable torch package that works very well for delicate work like this.

With the bottom edge fully tacked in place, I check the fit of the trunk lid one more time before everything is tacked and welded.  You can see how much better the gap looks even before the final finish work is complete.

Using my exceedingly rare and specialized gap checking tool (a standard 3/16" thick wooden stir stick), I check the gap along the entire forward edge making note of the spots that will require tweaking before welding.

Here, you can clearly see the perfect gap between the body and the newly added edge material.

This is a very useful welding accessory made from a 12" length of welding cable that has been stripped and twisted together.  This makes for a vastly improved grounding surface on any smooth metal panel and also acts as a cushion for the ground clamp on any welder.  Make one!

This is how the grounding aid is used.  Note how the copper bundle acts as a "cushion" for the ground clamp while maximizing the grounding of the work.

Another worthwhile accessory that is worth its weight in gold!  The Tig Finger is a nifty invention available for about $12 at Weldmonger.com that allows you to slide your hand directly over a fresh weld bead without getting scorched.  Worth every penny!

Here, the top edge has been fully brazed.  Note how contained and small the heat affected zone is along the entire edge.

You can see how minimal the weld bead material deposit is along the edge.  This makes things very easy to clean up with the grinder.

After dusting over with the air grinder and a 60-grit disc, the edge blends perfectly with the base metal with absolutely no porosity or unfilled gaps.  This is a distinct advantage of using TIG-brazing with silicon bronze filler on applications like this.

After smoothing up the edge with some 80-grit on my DA sander, the panel gap is checked one more time.  Looks good!

I did find a slight bit of warpage in the corners of the lid after all of the gap work was completed.  This was due to the fact that the trunk skin was not tacked to the support structure like a factory lid would be.  This allowed the edge to crawl a bit and add a slight wave to the metal.  I marked these spots with a marker to allow me to accurately shrink these spots with a torch to bring them back to shape.

A little careful shrinking with my torch and the panel pulled back very close to where it needed to be.

A few very thin applications of filler were all that would be required to get the panel dead flat.

Ready for primer!  The filler in this shot looks much thicker than it actually is.  Most of the filled surface is so thin that it is almost transparent.  Not bad for all of the work involved!