For what you are talking about doing, the only reason i could think of for tigging it is if you are desperate for practice. Mig is faster and easier and requires less prep. Just my two cents. Sometimes good enough is better than perfect. In this i think its true.

 

MIG would be my choice

 

Are you pulling the battery box out for repair, or is it integral to the body of the car?

If integral, how are you gonna get your tig torch in that area and see the puddle? How would you be able to dip filler rod?

MIG all the way.

 

If you can get the Tig torch in and work the wire you could get a smother weld and not have to grind back as with the Mig welder I would think.

 

Probably, but the time you would save with a mig and a flap disk would hardly make it worth while.

 

This is a hobby weekend car for me, so not concerned about how long TIG might take compared to MIG, but it may be hard to get in there. The battery tray is part of the unibody, so needs to be welded in situ....

This is in a place no one will ever see -- so what kind of welds should I use if MIG? Overlapping patch with lap weld? Form fitting patch and butt welds? Full bead or just spot welds with a seam sealer on top?

 

Dare I suggest neither welding technique? I'd be seriously tempted to use a well-formed patch to be pop riveted in with either 1/8" or 3/16" stainless steel rivets. No worry about setting the car on fire and no worry about frying the car's electronics. Stainless steel pop rivets are very strong and don't look too bad in case somebody does happen to look down in there.

 

Tig is very intolerant of contaminants like paint, rust, battery acid corrosion etc. Mig tends to be a bit more forgiving with stuff like this. I'd opt for mig personally because prep doesn't have to be quite as perfect as it does for tig, as well as it's probably easier to see what you are doing with mig.

As far as how to go about the patch, seeing the problem would help. Most likely I'd just oversize the patch and lap weld it.

 

I would cut a piece of 12-18 gauge sheet to size and lap weld tacks only. Easier to cut out if needed since you won't be able to paint the bottom after welding.

 

did you ever tig this project?

 

Being uni-body the patch should be welded all the way around in order to maintain structural integrity. The choice to butt or lap weld depends mostly on your experience level. I personally would definitely go with the mig, it will allow you to fill in any blow through that almost always happens on 18 ga. which is likely what it is. If it is removable by cutting out a few spot welds that would be the preferred method. Keep in mind that any lap joint can allow moisture and salt to get between the layers perpetuating the issue....Seam sealer might just peal up from the acidic environment. Imho, pop rivets will only create a place for salt to collect and cause more grief, unless you are lucky to live in the snow free areas. Good luck with your repair.:drinkup:

Does your parts car have a good box?