Repairing the trunk drop offs is a standard part of Mustang repair. If you look at modern cars, they use plastic for the front and rear drop off area, also newer cars do not seal this area off, effectively trapping moisture and debris. As you can see in the picture on left, my mustang is pretty typical, the lower area has rusted out, and at some point a prior owner pop-riveted some metal in to cover the hole. This was followed up with a liberal covering of silicone sealer. All of this created more area for moisture to stay, and actually promoted rust.
At least this is a somewhat easy fix, as there are drop down panels reproduced. The first step is to drill out any spot welds that are still holding the drop off to the quarter panel. I cover how I used a thick cut-off wheel to remove the spot welds in the post on removing the quarter panel.
With the drop off loose, I used my cutoff wheel (with a thin disk) to carefully cut along the bend where the trunk floor becomes the drop off. You can see the cut below, sorry i did not take very many pictures of this step, the cutting went pretty quick and I just forgot to stop and take any pictures.
Once the old drop offs were removed, I sanded down any remaining surface rust, and removed some missed under coating down by the frame rails. Above you can see a test fit of the new metal. I used the lip on the new drop off to provide enough surface area to attach the two parts. With all the fitting and trimming out of the way, I sanded the new part and coated it with weld through primer.
I plan to use Fusor 108B, which I have used is several post in the past to attach the new drop off to the trunk floor. Since this is not an easy part to clamp, I drilled an 1/8′ hole in the trunk floor about every 4″ to ensure that I had a good uniform clamping pressure. Next I mocked the assembly up with clecos (See pictures 1,2 above). Then using a marker I traced around any area that would have adhesive applied. I then went back and sanded those areas to bare metal to ensure a good clean surface for bonding. The areas I chose to bond were the wheel well, and the lip on the drop off to the trunk floor. I’m going to weld the trunk floor brace (The triangle shaped part in photo 1 of the first set of four pictures.) Since I did not want to trap moisture inside. I did apply internal frame sealer to the inside of the trunk floor brace before attaching the drop off.
In Photo’s 3,4 above you can see the bonded areas, there is good coverage of the adhesive. Once it cured, I removed the screws and welded up the holes. The Fusor stuff is great as it is designed to support combination adhesive and spot welds. The instructions cover two techniques, Wet or Dry. I chose the dry steps since the screws were supplying the clamping force.
Finally I ground down the welds flat and applied seam sealer to any of areas where water my splash or get to. Next time, I’ll post about painting the trunk area and preparing for installing the new quarter panels.