Datsun Derrière

I actually did this job some time ago, but figured it may be of interest, as it seems many quite capable car nuts are unwilling to tackle the replacement of these large panels, often preferring to just repair the wheel arches on their own instead. If you can get hold of the panels, there isn’t actually much more work involved in replacing the entire quarter panel, and of course you need virtually no body filler either. Replacing the entire quarter panel also allows you to rust treat and paint areas that wouldn’t normally be accessible. Once you have new rear quarter fitted, you can apply plenty of cavity wax and be certain that there’s no longer any rust lurking, and the car will remain solid for years to come. It does seem like a daunting task, but in reality it’s no harder than replacing any other welded on panel, much like a sill. Here’s how it’s done…

First… the patient. My old 610 Datsun 180B saloon (now sold) was a pretty solid car with only 62,000 miles on the clock, but there were signs of previous repairs and rust beginning to poke through on the rear arches. Whilst replacing the entire rear quarters for such a small amount of rot might seem extreme, it actually makes a lot of sense. Rust like this comes from the inside and works its way out. At this stage, it probably hasn’t done too much damage to the inner arches which saves a lot of tricky fabrication work. If this rust was just patched up and hidden, by the time it’s progressed to the point where it really did need doing, the inner arches would be in pretty poor shape, and those are the panels you usually cannot find. Fabricated repairs to inner arches involves a lot of double curvature metal and is difficult and time consuming to do, so it’s best to eradicate the rot at an early stage.

NOTE: You will need to support the rear of the car on axle stands and remove the rear wheels to allow access to the wheel arch seams. Support the car under the rear cross member or if the suspension is removed, under the crossmember mounts. Do not try to support the car anywhere aft of the rear axle center line and once the rear quarters are removed there will not be much strength in the back end!

To get the car ready for the job it obviously needs some parts removing. The rear bumper and lights, any lining in the boot and the fuel filler neck all need to be removed. It’s vital that the open filler tube to the tank is closed off to prevent any stray sparks from entering! At this stage, leave the boot lid untouched, as it’ll be useful later for aligning the new panels. The trim inside the car around the rear screen and the parcel shelf needs to be removed and finally the rear screen has to come out. This is bonded so will need to be cut out. You can do this with a sharp knife by repeatedly cutting along between the frame and the glass from the inside of the car. You can get specific tools for cutting out bonded glass, but with care you can do it with a knife. Once you have cut as deep as you can around the inside, go and cut around the glass from the outside. Patience is the key here.I didn’t actually remove the glass until after I had chopped away the bulk of the bodywork but this isn’t advisable as glass is very easily damaged by grinding sparks.

Once the car is stripped out ready, the first job is to locate all of the spot welds holding the car together at the seams. Around the window aperture, you’ll have to remove the remaining bonding adhesive to see them. The rear quarters are spot welded all the way around their edges and to the rear panel. If you are replacing the rear panel as well then that’s just spot welded across the boot floor. Is often easier to clean along all the seams with a wire wheel in a drill to make the welds more visible. The trickiest place is around the arch itself as it’s often a bit crusty there, making the spot welds less visible.

NOTE: Where the rear quarters join the rear panel, there as two small triangular panels, one in each corner. These DO NOT come with either new quarters of the back panel, so you’ll have to re-use the old ones. It’s worth drilling out the spot welds that hold these in place and removing them before attempting to remove any of the panels. If you are not replacing the back panel you can just leave them in place attached to the back panel. Likewise the fuel filler housing will also need to be removed from the old rear quarter and welded into the new one.

Having cleaned up the seams to make the spot welds visible, I usually go along and mark them with a black permanent marker or a paint marker, so they are clearly marked for drilling out with a spot weld drill. But before attempting to separate any seams, it’s easier to chop off the bulk of the panels first by cutting with either a small angle grinder or air chisel, along next to the seams, leaving the spot welded edges intact. If you remove the panels this way it gives you better access for separating the spot welded seams after, especially if you are using an air chisel.

Where the rear quarter meets the roof, the quarter panel actually goes up inside the roof skin so clearly it won’t be possible to fit this part of the new quarter. So this area will need to be carefully cut and butt welded. Before you can cut across the pillar you’ll need to remove the factory body solder by heating the area with a gas torch. Once the solder begins to melt you can quickly brush it out with a wire brush. Once the seam is visible you can cut carefully across just below the original join to separate the panel. Make this cut nice and straight as you are going to have to cut the new panel in exactly the same place to get a good joint for welding

With the panels gone, you can get to work removing the remaining seams. Where they are accessible, you can drill out the spot welds with a proper spot weld cutter. This will allow you to cut just through the top layer of metal leaving the seams of the inner panels intact. You can also use the air chisel or even grind away the top layer with a small angle grinder or die grinder. Once the last bits of the old panels are gone you can clean the seams up ready for welding the new metal in. At this point, any fabrication needed for rust repairs to the inner arches or boot floor can be done, and finally a coat of paint slapped over the entire inner surface. I used POR-15 on this one.

The chances are your replacement panels will be new old stock and therefor not in perfect shape. I usually prep the panels by sanding off the factory primer and treating them with something like Eastwood’s Metal Ready to dissolve any surface rust. I then paint the inside of the rear quarters with POR-15, but the inside of the rear panel is just primed with normal etch primer. Unless you have a spot welder, you will have to punch or drill holes at regular intervals along the seams to allow you to plug weld the panels into place using your mig welder. Everywhere where the old panels were spot welded will need to be prepared in this way. Make sure the seams are still straight and flat if you drill them, with no burrs on the edges of the holes. Using a punch is a far better bet for doing these holes. If you have a compressor I strongly recommend buying an air powered punch and swaging tool. It’s a real time saver!

Where the rear quarter meets the roof you’ll need to cut it to line up with the cut you made removing the old one. You can usually do this by cutting the excess metal from the top of the new quarter (the bit that would normally go up into the roof) and then offering the panel up in the car, overlapping to join so you can gauge where to cut. You might also be able to achieve the same by just measuring from the step in the panel to the cut. If you accidentally cut too much off so the gap is too big you can remedy it by welding a strip in behind to join the two panels. This area is going to be soldered anyway so it’s not really necessary for the joint here to be pretty but it does want to be strong as it’s a structural area. When the panels are ready, fit them in place and hold them with clamps (lots of vice grips are best). At this point you can close the boot lid to make sure the gaps are good and everything aligns as it should. When you’re happy with the fit then get welding! With plug welds, it’s best to use quite a high power setting on your Mig to ensure good weld penetration.

When you’ve done welding you’ll need to solder the joint between the quarter and roof. If you’ve never done this before just buy a kit with some instructions and have a go. It’s not that hard really! The basics of applying body solder are as follows. First you have to ‘tin’ the area to which the body solder must adhere. This is done with a solder paste which is spread over the area then heated with a torch until it melts. The paste is made of powdered solder and flux. The flux cleans the metal so the solder can bond to it. Once the area is tinned you can then start applying the body solder. You need to apply just enough heat to get the stick of solder to melt to a paste-like consistency which can then be spread out over the area using a wooden paddle dipped in tallow. Once it’s cooled you can get it roughly to shape using a body file and finish it by sanding.  Note that it’s really not wise to breath solder dust from sanding, so wear a mask and take care! Note that you’ll need to make sure you get enough solder into the joint as necessary first time, as unlike body filler, you can’t easily add a bit more later if it’s too low. If you file it and find a low spot you’ll have to melt the lot and re-apply more!

Sand down all the plug welds you have made along all the seams with a grinder (flap wheels are best for this) and finish by sanding the entire panels back to shiny bare metal with an 80 grit disc on a DA sander. Then you’re ready to start the painting process. Once you have your paint on you can rust treat all the seams and cavities ( I use Dynax S50 from Bilt Hamber) and know for certain you’ll have no rear arch rot for years to come. Incidentally. if you have used POR-15 inside of the panels and want to coat them with something else, either stone chip or paint, then you can buy another product from the same company called Tie Coat primer, which is specially designed to promote adhesion between POR-15 and other top coats. You can use seam sealer along the inside of seams but I prefer not to as it can trap moisture and effectively hides any ensuing rust until it’s quite advanced. Better to protect the seams with wax and be able to see they are rust free!

The finished bodywork waiting for paint…

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