Trunk Lid Finish Work & the Beginnings of Quarter End Cap Gapping

It’s been a few weeks since our last update but things have been busy here in the shop and I am happy to report the trunk lid work is done and the results are superb.  Continuing the thread from our last update, the trunk lid required a bit of careful filler work to get it perfectly flat and ready for primer.  For the first round of filling, the work began with our old standby filler, Evercoat Rage Gold. 

Rage was applied in very thin, smooth layers and block sanded between each coat until any traces of imperfections were gone.  This is a very iterative and labor intensive job as the idea is to use as little filler as necessary to get the panel straight before moving on to high build polyester primer and fine finishing putty to get the  final shape perfected before applying epoxy primer/sealer.  This is the kind of work that requires plenty of patience, rock ‘n roll, and discipline with the sand paper to ensure you don’t gouge the surfaces and thereby create more work for yourself.  The key is to refresh the paper on the sanding blocks often and let the sandpaper do the work.  Don’t “muscle” it with the idea that you will get done quicker.  Ain’t gunna happen that way.

With the Rage filler taking care of the vast majority of surface correction, the time came to lock the surface down with a few thin coats of PPG DP40LF epoxy primer mixed as a sealer by adding about 15% reducer to the mix.  This would seal up the entire surface of the trunk lid and allow us to finally get a critical eye on the panel surfaces when all in a single color and with a consistent surface.  After a solid overnight drying period, we were rewarded with a very straight panel that was ready for the finer detail finishing that high-build poly primer and finishing putty would bring.

With the primed trunk lid surfaces thoroughly dried, the entire surface was treated to a rather aggressive scuff sanding to allow the PPG Shopline JP205 high-build polyester primer to grip the surface as aggressively as possible.  JP205 is essentially a “sprayable” and extremely high quality polyester body filler that is applied evenly over the entire surface of a panel and then very carefully block sanded to achieve a laser-straight surface.  This stuff might just be the closest thing to magic in a can that you will ever apply to a custom car body surface.  With a film build of only about 3 mils per coat, you can very efficiently apply a workable surface to the entire panel that is very easily sanded to virtual perfection in very short time.

In our case, we started with two moderate coats of JP205 and allowed this to cure overnight.  Then, the panel was carefully block sanded with the longest boards we had to carefully level the surfaces yet again.  With the darker DP40LF epoxy primer as a base, it was very evident that we had a few very, very subtle imperfections that we needed to address.  And this is where the high-build primer really shines.  By using lots of dry guide coat and 120 grit sand paper that was changed often, we were able to hone in on the specific areas that required more attention with extreme accuracy.

A second application of two coats of JP205 was all that was needed to completely eliminate the few remaining hiccups in the surface following a very deliberate and careful sanding exercise.  The final finish work was accomplished with a new product from Evercoat called Ever Gold finishing putty.  This is a brand new product in the shop and I have to say I absolutely LOVE this stuff.  It lays on exceptionally smooth and sands and feathers out to gorgeous finish.  This was used to correct the last tiny specks and pinholes that remained in the surface.

Finally, the “beauty” surface of the trunk lid was treated to two thin coats of PPG epoxy primer/sealer and allowed to dry for a few days before transitioning to the prep work required on the bottom side of the lid.

The only corrective work required on the bottom trunk lid surfaces was to tidy up the forward flange area that was welded up in the gapping process.  This was very easily addressed using the Evercoat Ever Gold finishing putty to smooth the grinding marks and create a flange that is virtually indistinguishable from a factory flange.  In fact, unless you followed the modifications here, you would never know the extent of work that had gone into this forward trunk flange.

Next, the entire bottom surface was scuff sanded in every nook and cranny in preparation for a few coats of PPG epoxy primer.  I really like using the 3M Scotchbrite “red” pads for a lot of this type of work as they are very flexible and durable and provide a great surface for primer without gouging up the metal in the process.  Then, the bottom surfaces were thoroughly cleaned and tacked off and two thin coats of PPG epoxy was applied to seal things up.  And with that, the trunk lid work was complete and the results look absolutely awesome!

Next up, we started the gapping process on the rear quarter end caps.  This is an area where I will divulge a little secret that can make a big difference in how a car is perceived at a show or cruise.  More on that in a minute.

Quarter end caps are a bit of a love/hate relationship on Mustangs.  In the case of the 69-70 body, these caps are die cast alloy and more often than not, do not match the exact contours of the quarter panels they are bolted to.  Complicating this is the fact that they can also damage fresh paint on the rear of the quarters if they are tightened down too hard and the factory gasket device is compressed.

Nine times out of ten, the end caps are hard against the rear quarter flanges when mounted to the body, leaving no “gap” at all.  I have always found this to look a tick bit odd when all of the panel edges around this part have…..well……gaps.  So, to remedy this perceived error (and here’s the secret), I carefully shim the end cap out away from the end of the quarter panel using simple hardware washers until I have a consistent and precise .047” - .050” gap across the entire flange.  This subtle modification creates many beneficial features.  To start, by creating this thin gap, the need for the rubber “seal” is completely eliminated and the potential to damage the paint as a function of securing the end cap is virtually eliminated.  Secondly, moving the end cap slightly rearward on the quarter allows more room to blend the end cap contours into the rear quarter for that extra detail that everyone notices, but rarely can put their finger on.  And finally, it allows the rear quarter flanges to be worked perfectly flat to match the end caps with precision and allows the end cap mating edges to be precisely fit to the quarter to give the quarter a cleaner, more flowing line all the way back.  The difference is rather striking for such a simple modification.

In our next update, we will be finalizing the end cap gaps and machining the final spacers that will be used to maintain these gaps with more durability and precision than the washer stacks will allow.  Then we will document the work going in to contouring these caps to perfectly match each quarter for that “super finished” look.  Also, we will start cleaning up the lower valence panel end contours and blending them into the lower quarter sections for perfect fit.  Lots more to come!

Several thin applications of Rage Gold filler leveled out the trunk lid very well.  The darker edges are actually the underlying base metal showing through the almost transparent feathered edges of the filler.  A good indicator of just how thin the filler actually is.

Sealed up in a few thin coats of PPG epoxy primer/sealer.

The PPG epoxy primer was left to cure for a few days and then scuff-sanded in preparation for a few coats of JP205 high build polyester primer.

PPG Shopline JP205 high-build polyester primer is essentially a sprayable body filler.  This stuff is rather nasty looking when applied but it blocks out beautifully and is a key in getting a panel laser straight.

Almost done blocking the first application of JP205.  This shot is very telling as it clearly shows the few bare metal high spots that were revealed that could not be felt with the hand, the darker grey slightly high spots where the epoxy primer was revealed and the light grey band about 10 inches up the deck lid that shows the guide coat in the area that requires a tick bit more work.  Lots of value in this shot for sure!

Another look after the first coat of JP205 was blocked down.  The back side of the trunk lid was surprisingly straight and required very little work to get dead flat.

I have been trying a new product from Evercoat called Ever Gold finishing putty and so far, I love the stuff!  This is an excellent fine filler that easily handles tiny imperfections like pinholes and sand scratches.  Sanding is a breeze and the finish is excellent.

These thin skims of Ever Gold were all that was needed to correct the sanding scratches and few pinholes we had .

Prep work for the second coat of JP205 high-build primer is complete and the panel is being tacked off before spraying.

The final coat of JP205 is on and allowed to cure overnight before final block sanding would begin.

Here is the trunk lid after final blocking on the JP205 high-build primer.  A bit of cleaning and tacking and we are off to epoxy primer/sealer!

All sealed up in PPG DP40LF epoxy primer/sealer wet out of the gun.  You can get a  idea of just how flat and beautiful the trunk lid is based on the reflection of the door frame in the surface.  Once this dries, we can tidy up the bottom surfaces and get them primed too.

In this shot, you can clearly see the silicon bronze weld bead along the edge where material was added to tighten up the gaps.  To ensure this modification would remain completely invisible, I decided to apply a thin skim of finishing putty along this edge to fill the grinding marks and any subtle dips that may remain after all of the work is completed.  The masking tape ensures I don't end up with a nasty edge along the back side of the flange when the filler is applied.

I went a little bit heavy on the finishing putty to give me plenty of material to work with when sanding this edge to shape.

With the welded edge smoothed up, I moved on to scuffing the rest of the surfaces to prep them for primer.

PPG epoxy primer fresh out of the gun looks fantastic, even on the bottom of the trunk lid where almost nobody will ever see.

Details are everywhere!  The modified flange looks like it came straight from the stamping press.

Quarter Panel End Cap Gapping Begins!

 

Rather than have the quarter panel end cap extensions mounted hard against the rear quarter flanges where they often damage the paint, I prefer the look of having a slight gap.  I start this work by shimming the end cap away from the flange using standard 1/4" flat washers.

I try to target a gap of .047" - .050" on the quarter extension/end caps for just the right look.

As soon as a gap is established between the end cap and the quarter flange, it becomes very evident how much work the quarter flanges will need to get truly flat along their edges.  It is amazing how much the quarter end caps hide when bolted up solid.

Where the gaps are a bit too wide (or narrow) I mark the amount they need to change directly on the part to allow me to correct it by the exact amount required.  In this case, I need to reduce the gap in this area by .020" to match the target gap.

Here is a section that will require slightly increasing the gap along a section about 3 inches long.

 

Once the end cap gaps are established, the look is surprisingly more refined.  A lot of builders like to mud these gaps closed, but that is a look I have never quite appreciated on the classic Mustang and is due, in most cases, to the obvious use of a lot of body filler to achieve the "seamless" look.  Nope.......not on this project.

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!