I have several spot-welded patches over weak or missing areas in the sheet metal of my ‘63 FHC. For example, these photos are the outside and inside of a door patch:
Drill out the welds, then MiG weld the holes.
A welding trick for patching holes in thin metal is to place a block of copper behind what your welding. The weld won’t stick to the copper and it gives it a backing so you can puddle across openings.
check Eastwood for a selection of copper tools for this purpose
Practical cost effective .time 1,5,10,20 years how’s the rest of the car what’s your skill set ,how much 
do you have
It’s a big rabbit hole your looking at and that’s not a bad repair in the big scheme of things
The proper way to fix it is to cut it out and weld in new metal.hammer weld ,tig ,or mig stitch
Anything else is well what you already have so.
They make a filler with metal in it for these jobs
Fill rust holes from the back , after lowering the patch with a hammer and dolly use regular filler on the out side shape it with 40 gritt to start ,80,160,320 dry and a 10 inch block
It will out last your ownership done right
Generally speaking
You’ll need some filler anyway if you want it laser straight
Learn the skills it’s fun
Thanks, everybody! 17 18 19 20
If you use thin, eutectic bronze rods, and know how yo ‘run’ the brazing into the rust pits, and inly use a small flame, yes, it can work.
if the backside of that patch is well attached, I would just weld or braze in the outer seams, smooth it with a small bit of either lead or plastic body filler, and then feather up to the metal.
Is an inly small flame the one that goes pop when too inly 
Ya the tip over heats not enough flow to keep it cool 
other reasons to
Hello Paul,
Any heat that gets the metal to a red, plastic state, as is the case with brazing, will normally result in distortion; the metal in the hot spot will shrink. Due to the resistance of the cold, surrounding metal, there is nowhere for the expanding metal of the hot spot to go and the plastic state of the hot spot causes the metal in the hot spot to compact. When it cools, the area of the hot spot will have shrunk.
The old school way of repairing deep rust pits or small holes, if generally the surrounding metal is in reasonable shape and there is not a great number of pits or holes, is to:
use a twist drill large enough to create a chamfered hole out to the boundary of the pit
using a soldering iron with a large copper head, the one I have has circa a 40mm square head, drawn to a very small point, tin the chamfer of the hole
using 50/50 solder wire (its harder than wiping metal), with the point of the soldering iron placed in the hole to heat the chamfer all around, touch the solder on the interface between the soldering iron and the chamfer.
the solder will flow to the tinned surface and with a twisting action of the soldering iron on withdrawal from the hole, the surface of the solder deposit will be slightly convex.
file the solder down to the surface of the surrounding metal.
Back in the 60’s, it was popular to remove the chrome strips that adorned cars to make a plain, clean look. If the car was already painted, the technique of chamfering the hole and filling with solder as described above, could be carried out with zero damage to the surrounding paint. The solder was filed down to the surface of the paint with the area to be resprayed kept to a minimum. The most common paint on cars in those days was Acrylic Lacquer, which allowed for paint repair without painting the whole panel.
Regards,
Bill
Soldering is most certainly a good technique: that said, my dad taught me at a young age how to use thin eutectic bronze and a very tiny flame to patch relatively thin body metal with very little distortion.
