Rear Body Detail Work & Hi-Build Primer Application

They say that “slow and steady wins the race”.  However, for a guy who has been in racing and performance cars all his life, THAT is a very LARGE pill to swallow.  A pill roughly the size of a men’s size 14 Reebok Hi-Top in fact, but as wiser men than I have counseled over the years, patience will pay off and I trust in that.

With the end cap gaps set earlier on, it was time to invest a few hours leveling the remainder of the end cap surfaces.  Oddly enough, these die cast alloy parts are actually very wavy and rippled and require quite a bit of massaging to get them smooth and contoured correctly.  In fact, just about every surface of these parts requires a thin application of filler and repeated blocking to get them straight.  Fortunately, the end caps I have were reasonably good and very little filler would be required to get them in top shape.

Next, with end cap and valence gaps looking very tight, a few hours was dedicated to getting the forward edge of the trunk lid gapped exactly as I wanted and the contour matched to the trunk lid.  Since the trunk lid and lower rear window panel will maintain the classic Boss 302 trait of flat black paint, these edges need to match well to not look out of place. 

With very thin applications of Evercoat Ever Gold glaze, the surfaces and edges of the trunk edge were brought up to match the trunk lid edge very well.  With a bit of hi-build polyester applied, these areas will be able to be perfected with careful block sanding.  Also, I permanently glued specially machined spacers to each end cap mounting stud that ensure a precise gap is maintained.

Following the end cap work, we transitioned to the task of tidying up the flanges that mate to the quarter end caps.  As stampings, these surfaces were anything but straight and flat and the factory made up for this by using a thin rubber gasket between the end cap and quarter.  Since these end caps have been custom fit with a thin gap, these gaskets no longer work (or even exist) in the project.  So, extra time must be taken to carefully fill and flatten these flanges to ensure the finished gap is straight and even from top to bottom.  Yep, more work that you will never see, but the end result is what it’s all about!

The focus was then shifted to clean up the flanges on the back side of the rear valence to ensure the panel gap is properly presented and the edges of the panel are clean.  I prefer to use a product called Duraglas by USC for this type of work as it is completely water proof and works very well as an edge reinforcement with its fiberglass-strand reinforcement directly in the mix.

The final detail work to be completed before masking for hi-build polyester primer was to smooth the rear fender arch flanges to complete the look we are after in the detail areas.  Most factory Mustang fender flanges are nasty bits of jagged metal and lumpy spot welds that look awful with even the finest finish applied.  A lot of cars have fender opening trim that somewhat hides these flanges, but Boss 302’s never had that crap on them and therefore, these had better look tight!

Moving forward, the time had come to scuff all of the rear body surfaces with red Scotch Brite and 150-grit paper to provide plenty of “tooth” for the hi-build primer to grip.  Now, it was time to start the slow process of masking every edge at the rear of the car to prepare for the application of two medium coats of PPG Shopline JP-205 polyester high build primer. 

I have come to love the convenience and time-and-material savings offered by the use of a professional masking machine for masking jobs like this.  A masking machine is simply a device that applies masking tape along the edge of a roll of masking paper and has a cutting edge along the bottom to allow the user to quickly tear off the desired amount of pre-taped masking paper.  All told, I reckon this device saves between 50 and 75% of the time that would be required to apply masking tape to the masking paper by hand.  Worth every penny!

Finally, the application of two generous coats of PPG Shopline JP-205 was upon us.  As primer products go, JP-205 might just be the closest thing to magic in a can there is!  Essentially, hi-build primer is a sprayable polyester body filler that is easily sanded.  By coating the entire panel in a thin, even coat of filler, the panel can be virtually perfected with every contour and body line capable of being as crisp and clean as the most perfect example anywhere.

Like regular body filling techniques, applying and sanding hi-build is an iterative process that relies on careful blocking with guide coat to ensure that every panel detail is perfected.  Once the body is essentially perfect, the hi-build is sealed with epoxy primer and the plans for base color and clear can be established.  Nope, this ain’t no “collision shop” operation here!

On what might end up being the last decent day of what remains of 2014 here in occupied Southeast Michigan, all of the subject panels were squirted with hi-build and allowed to dry for a few hours before the masking paper was removed.  The end result was a nice, contiguous base that will be ready for slow and meticulous block sanding to flatten every surface and them more and thinner applications of hi-build until the entire works is as close to perfect as I can manage.  However, in the meantime, we will let these surfaces cure for a few weeks while we begin to evaluate door hinge rebuilds, trunk hinge powder coating and some seam sealing.

Happy Fall everyone!

The die cast end caps on my 70 are actually pretty good examples.  But plenty of filling and sanding is still required to get them straight and smooth.  Evercoat Ever Gold glazing putty is my filler of choice for detail work like this.

 

Ted blocking out the end cap after a thin coat of glazing putty.

 

This shot really gives you an idea of how much work goes into getting all of the rear contours just right and how much variability there really is in the "factory" rear bodywork of these old cars.

 

The left side was actually a wee bit better overall than the right, but still requires a lot of work to get into shape.

 

With all of the detail filling work done and the contours smoothed up, the added character of the thin gap we added between the end cap and the quarter flange can really be seen.

One area that is often overlooked at the rear of 69-70 Mustangs is the outboard transition area between the lower rear window closeout and the rear quarters.  Most cars have a dip in this area as shown by the straight edge above.  A bit of filler is added to this low area on each side and carefully sanded to bring the surface level.

Here is the same area after filling and sanding.  No gap anywhere along the length of the straight edge.

 

Here is another look at the transition area without the straight edge in the way.  Very little filler is required to get these areas right.

To ensure the tight gaps we added to the rear quarter end caps are precise, I machine these custom spacers to the exact dimensions required to maintain the exact gaps every time the end cap is removed and re-installed.  Each spacer is number to match its individual stud as indicated by the numbers on each spacer and at the base of each stud.  Then, each spacer is epoxied to the base of each stud and installed on the car to dry permanently.

 

In this shot, you can see the spacers glued into place with industrial epoxy. 

 

The rear quarter flanges also require careful attention to ensure they are flat and square to match the end caps perfectly.

 

 

This shot shows how much work is often required to get the rear quarter flanges flat and straight for proper panel fit.

 

Using USC's Duraglas fiberglass reinforced filler, I even up the flanges on the back side of the rear valence to ensure the gaps are consistent when the panel is installed on the body.

Before masking, every panel gets a light scuffing to give the PPG Shopline JP-205 primer good "tooth" for adhesion.

Using a flexible sanding block with 150-grit paper, Ted carefully scuffs the inside radius areas of the quarters in preparation for priming with hi-build.

 

We start masking from the rear tail light panel forward in prep for the hi-build primer.

 

A professional masking machine makes masking large areas of a car amazingly quick!  We are ready to shoot some hi-build primer!

Starting with the smaller loose parts, everything gets two generous coats of JP-205 Polyester High-Build Primer.

Even with the slightly rough texture of hi-build primer, the deck lid still looks great!

And there it is!  The rear of the car is finally in its first round of hi-build primer and is ready to be blocked.  Lots of work yet to do back here, but this is looking pretty good!