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Here is the trailing edge of the front wing. It has been the subject of numerous bodge jobs. There is corrosion around the attaching brackets which have all but disintegrated. Water has also been trapped beneath the filler against the metal wing causing rust. This shape is awkward to replicate because of its sharp double curvature. The bad rusty metal has been cut away, try to remove as little metal as possible even if this means a disjointed cut like the one shown above. However, don't make the cut hugely complicated otherwise it will be difficult to match the repair panel. 
Here is the  flattened profile for the repair panel. I flattened out what was left of the old rusty metal to give me and idea of what shape to make. Take measurements  from the surrounding panels and from the other to wing to develop an accurate idea of the shape to create. Alternatively, create the 3D repair section with little overhang and when you have found the correct shape just trim of the excess. Panel Beating: (the secrets)
  • Though out the shaping process the panel changes shape dramatically. It may look about right one second but with a twist or a tap suddenly it wont fit at all.
  • Panel beating takes time and patients. The quality of the end product is all ways a result of the time and effort expended.
  • Finishing a repair panel to an impeccable smooth surface without the use of a wheeling machine is near to impossible. It can be done but it takes far too long. (to remove hammer marks, either file down or skim over with filler)
  • Plasticine acts almost exactly like steel during the stretching and shrinking process. Get some plasticine and apply various stressed with your fingers. (squeezing, stretching, compressing, etc) Watch how it 'moves' under these stresses, try to understand why and how these forces cause the shape to change. This may all sound a little spiritual, but it helped me a lot in learning to work with metal.
  • You gota try and BE the metal... man !
When the panel is the correct shape, clamp it and tac-weld it. Confirm the repair panel is in the correct place. Weld the panel in using 2-3 inch runs allowing to cool between. Don't try and weld in one continuous run because the metal will be over heated, expand and when it cools down it will contract and buckle.

TIP: Welding the panel in whilst mounted on the car allows me to constantly assess door gap and fixing position alignment.

Here we can see the front inner wing bracket has been attached. Very simple just two little spot welds. 
There is another little bracket welded to the underside of the rear trailing edge. There rain is on its way so I have coated everything in primer just in case. A small fillet has been welded to the inside of the wing. This fillet has two purposes. It gives strength to the bottom part of the wing and carries a rubber sealing bead which would other wise be loose.
All welds have been ground down and a two-three coats of filler applied. Use a flat clean metal spatula to coat the large curving surface. (don't use the plastic spatulas supplied with the filler because there useless.) At the wheel arch try to use the spatulas but sometimes the best tool you have at you have at you disposal are your fingers, thumbs and the palm of your hand. TIP: Here is an example of how filler should be used. Use thin layers and feather out over a large area. Don't use huge amounts of filler. Don't run filler right to the edge of panels. The panels will flex and the filler will crack at the joint.
 
Files and rasps can be used for initial shaping. When sanding back filler try to use a sanding block. A solid flat piece of wood which the sand paper is wrapped around.  This helps maintain an even pressure on the surface and prevents the sand paper from digging in. (this is where the dynamic of the panel changes and the sand paper cuts in at too sharper angle causing a rut) A sanding block is appropriate for curvatures and flat surfaces alike. For sharp curves and  fiddly bits like the arch, there is no better tool than the various curves on your hand. 

When sanding down the surface in preparation for filler primer it is advisable to sand down to at least 800 grit. Top coat requires at least 1200 grit wet and dry.