I'm working on the interior of a classic Mini, and I'm trying to fill some 1/4" holes in the dashboard sheetmetal. I gave it a whirl with my TIG, but I'm just burning holes due to a lack of skill, awkward position, little light, trying to lift-TIG, etc...

I also tried to solder the holes, but using a torch in there gets real close to a bunch of plastic and rubber, and the resin-core solder I use for electronics was difficult to get to behave on the sheetmetal...and I don't know what I'm doing.

Is there a way to solder in tighter quarters? Maybe a little butane torch? Giant iron? What kind of solder should I be using for this? It's not easy to get copper behind the holes, though I could spend more time on it if this is the way...The videos I've found all say different things, with some people using lead, some people using iron, some using torches. I guess I need autobody soldering 101.

What if I just bought a cheap MIG welder, did a few tac welds to fill the hole, and sanded it smooth?

You cant weld over anything thats Soldered or Brazed, So all of that needs to be removed and the metal cleaned first. 80 grit or courser say 36-40 grit on a 2inch Die grinder pad. front and back metal has to be spotless clean metal before welding No paint no oils no solder no brazing just clean bare metal. I always do a Final wipe with Acetone b4 welding.

You really should not attempt filling a 1/4 hole with weld bead you should make small patches and weld them in. Can a Mig do it ? Yes is it right way? NO!

I suggest getting and old fender or hood from a scrap yard and welding on that first.
IF you have a Scratch start Tig you have the right equipment to do the job its just time to take the time to learn how to use it.

My best recommendation for learning to Tig welding is to learn how to Gas weld first in my opinion, learn how to lay beads with Oxygen & acetylene first its cheaper to learn how to control filler rod and Torch control that way. Laying beads on Auto sheet metal will be challenging as its so thin, It would behoove you to get a bunch of 3/16" and cut that up to learn how to weld on first.

Me myself I have a whole pack of Tungsten thats sharpened ready to go and just change it out after dipping. That way I always have a clean ready to go Tungsten then latter I can sharpen them all at once.
I also use a Product called Chem sharp for sharpening I don't grind my Tungsten.
https://www.youtube.com/watch?v=SSSaJ-Ke7as

Tig is very precise and every thing must be absolutely surgery clean, Having to clean and resharpen your Tungsten every time you dip it into weld/Bead pool gets old quick.
If you dip your junk you got to clean your junk! every time! I know its frustrating while learning. You cant weld with a dirty Tungsten tip! The Arc will wonder or be offset and wonder when its dirty or contaminated.

For a 1/4" hole I would use Silicon Bronze with a tig. Just hold the rod on the edge of the hole and heat mostly the rod, but the metal needs to be a little warm also. What you want is a drop to fall in or on the edge of the hole, but not to flow out. Then just hold the flame directly on the drop only, until it flows. You don't want the metal molten, just warm enough to accept the Silicon Bronze. By heating the drop only, it will become molten, but won't flow until the metal is hot enough. If it doesn't completely fill the hole, then let it cool and do it again. You can actually fill a larger hole the same way. Practice-practice-practice first.

When done properly the back side will be smooth and not need much if any attention.

chevman is right, as 1/4" is a bit large to freehand fill, or even use a copper backer. For this size or larger, I either cut small circles, or if there's a lot, buy them, and use them to fill. If that's too small a circle to weld, you can use a Unibit and make them a bit larger, and use larger filler circles.
I keep several sizes of these in the shop:
https://www.amazon.com/totalElement-Ste ... _ba_s_10_t