Here we are repairing some fairly extensive rust on a Maserati Biturbo as part of an overall restoration. The bottoms of all four doors need to be patched. This includes the inner doors as well as the outer skins.
I rough-cut the rotted areas to get an idea of how big the patch panel will need to be. Cut a piece of sheet metal a little oversize and fabricate the required features. These particular doors have a simple shape to the bottom of the inner structure and only need the pinchweld flange bent to the proper angle.
Somewhere in here the dirty work has to get done. I’ll clean the insides of the door with scrapers, wax and grease remover or lacquer thinner to remove all traces of sound deadener, undercoating, body wax, whatever, because that’s just the way it has to be.
I will install the doors several times during the process to test the fit of the outer skins before trimming the patch panels to their final dimensions. Whenever possible I like to lay the oversize patch panel over the area and then cut the panel and the original sheet metal to size in one step. I find cutting a straight line with a cut-off tool really hard to do so this is where a good plasma cutter pays for itself in saved time and reduced aggravation.
The patch panel and the original metal need to align without significant force having to be applied. The price of a poorly fitted panel is warpage and distortion which will need to be straightened later on. I always have to remind myself not to take shortcuts at this stage.
And now, onto the welding; I learned the art of hammer welding many years ago and am constantly amazed at how well the technique works. Of course the method was developed when automotive sheet metal was considerably thicker and access to the backside of panels was considerably better but is still to my way of thinking the best way to join two pieces of sheet metal. I also really like the whole oxy-acethelyne ‘thing’. The flame, the sparks, the occasional 2nd or 3rd degree burn. It’s just so macho in a way that TIG will never be.
So, once the welding is done all the slag and oxidation have to be removed. I use the usual assortment of wire brushes followed by a good scrubbing (inside and out) with chemical rust remover. This stuff will remove the last of the welding residue and provide a phosphate coating to protect the surface from rust.
After all that I’ll move to the good old hammer and dolly/pick and file stage to finish the repair.rust