There's a handful of ways you can tackle repairing rust in your vehicle and all of them have their place. The most common would probably be cutting out the metal and MIG welding a patch panel in place. While this method is the easiest to accomplish, it can be difficult to blend the weld seam into the surrounding metal. I've done repairs this way for many years and they've turned out ok, but I've always wanted to master TIG welding patch panels and metal finishing the area for a seamless repair. I've recently begun switching a lot of my welding projects on Project Pile House to TIG and I've been having decent results. I decided to take it one step further and show how to repair a thin-gauge rotted lower door skin on a VW Scirocco door.
I'm currently juggling a LOT of projects at once AND I'm limited on space in my own home garage, so I opted to send my 1977 Scirocco restoration project to a friends shop for them to do the final dent repair, panel fitment, primer, and paint on the car. When they were stripping the old paint off of the car they found that the passenger door had some pretty substantial rust damage that someone had just laid fiberglass and body filler over. I decided to let the guys keep rockin' on the car and I'd repair the door skin myself.
If you haven't noticed, early VW Sciroccos aren't exactly the most popular classic cars, and they aren't growing on trees in the salvage yards either. Because of this I had to make up the patch panel from scratch. I first marked out the repair area with painters tape and used the angle grinder with a cutting wheel to cut along the straight tape edges. I then used the flap disc to carefully sand the top layer off at each spot weld and separate the outer skin along the bottom.
Since the bottom of the door has little to no curve to it, making a patch panel was pretty simple. I first made a paper pattern out of chipboard to match the damaged metal I cut out of the door. I then cut the pattern out of matching 20 gauge sheet metal. I took the flat panel to the metal brake and bent the bottom "pinch weld" area of the door. I left a little extra on that edge so I could trim it to match the original (left room for minimal errors on the brake).
Up to here most of the process has been the same for any method of welding, but when fitting the patch panel it changes a bit. When MIG welding a patch panel you want to leave a small gap between the patch panel and the original metal. This is so a little bit of the weld puddle can fill the small gap and the entire weld isn't ground away when finishing the panel. With TIG welding it is the opposite, gaps are the enemy. I like to use a file and slowly "sneak up" on the fitment of the panel. Test fit the panel and use a double cut file to take away a little at a time until all gaps are gone and the gap is tight and even around the patch. I then go over the entire panel with a sander and acetone to remove any grease or residue for a clean weld seam.
When TIG welding thin sheet metal you need to make sure you have a fine, sharp tip ground on a 1/16" electrode for as small of an arc as possible. I switched the standard WP-17 torch on the TIG 200DC to the WP-9 Mini TIG torch. This torch is MUCH smaller and lighter which makes it easier to control when running short, quick welds on sheet metal. The torch is rated up to around 90 amps, but I set the pedal to weld in the 20-60 amp range. I also like to use filler rod that matches what I'm welding. I ran some solid core .030 MIG wire off of the spool and used that to run my welds.
Any type of welding causes extreme heat that causes the weld seam to shrink and cause a low spot in your panel. The nice thing about TIG welding is that the weld seam is softer than MIG welding and it can be easily planished or hammered on-dolly to flatten it out and stretch the seam back into shape. I like to stop after each short weld bead and hammer the weld to relax the panel back to its natural shape. This also allows you to let the panel to cool a little in between each weld to avoid overheating the entire panel.
Slowly as you weld and hammer the seam you will connect all of your short seam welds and have a fully welded panel. From here you can use a body file or a sander with 36-40 grit to sand the panel and highlight any offensive high or low spots and work them into shape. Once the panel is satisfactory you can use the 36-40 grit to smooth any imperfections and work your way up to 80-120 grit on the DA sander. This should leave you with a finish that's acceptable for primer or a light coat of body filler. This method should require little to no grinding and is much more controlled. I have begun to prefer it over MIG welding a patch panel, but it isn't always quicker or more efficient.
With seam all welded how I wanted I rolled the corner edge of the door over and drilled holes in the patch panel matching the OE spot welds on the bottom of the door using an Eastwood Pneumatic Drill. I used the MIG 175 to lay the spot welds and a flap disc on the angle grinder to smooth them out. The result was a door that needed very little filler and weld joint that's nearly invisible on the inside as well as the outside of the door.