What is the size spot- weld cutter are you guys using. I'm looking at some Blair product. 3/8 seems to be the common size but it seems like a pretty big hole to have to weld back... thoughts?
Thanks

The hard part is getting all the weld cut. Welding a 3/8" hole isn't that much different than 5/16". Did try the Blair cutter, got 11 holes out of both cutters. Picked up a Princess Auto (Harbor Freight) cutter, same drill, same speed, got over 100 cuts form one cutter. Some bits work, some don't. Slow speed and a good pilot hole seem to help. Carbide cutters work OK, but can shatter. With the planets aligned and holding your mouth right, you can do a lot of cutting. If not, well...

auto body tech here.
the official recommended spot weld cutter size is 5/16.
you have to be exact to completely drill out the weld at this size.
a 3/8 is much easier to completely remove the weld. but as you posted it take more effort to fill those welds back up.

the cheap HF cutters work. but don't last very long before they are dull.
i have use blair rotobroch cutters for years with good results. $$$ but work.

What are you doing? Sometime you can use the holes(drill threw) for plug welds on new sheet metal. I use the 11279 but there are a lot of places that make them now.
https://www.eastwood.com/search/?q=spot+weld+cutter

Previous posts have about covered it. With the smaller 5/16" (8mm) you will have to chisel more of them apart (maybe most of them) which will usually distort and tear the metal more. The 3/8" (10mm) results in a larger hole, but is more likely to get enough of the weld cut. I have had the most success with the drill bit style of cutters. The 'toothed' ones frequently break the teeth due to the often irregular weld surface. It is very difficult to hit (and to stay in) the center of every weld. I am currently using Irwin's Turbomax drill bits to drill out spot welds.
www.irwin.com/tools/drill-bits/turbomax-high-speed-steel-straight-shank-jobber-length-drill-bits

I had to do some fairly extensive reconstruction on the cab of my '65 Ford flatbed... got the Blair 3/8" set. I was somewhat disappointed in the cutter life also, but it was a fair bit better than 5-6 holes per cutter! Maybe 20-25... but I think my one-speed air drill was a little faster than recommended... As the instructions explicitly state, keep it under 500 rpm and lubricate the cutter. Overspeeding will ruin them quickly due to excessive heat.

Interesting that the Harbor Fart set actually lasted that long. My town finally got a store this spring - I may buy a set and check it out

I agree that it's not hard to fill a 3/8" hole. Sometimes the 5/16" can actually be more difficult because it's hard to get it started and circling before the hole fills up!

All very good info. Thanks. I like the the Irwin drill bit tip...because of the shape of the tip. I have some similar spot -weld bits, but they are hard to control and many times blow right thru both pieces.

I've tried at least four different types of cutters and the Blair 3/8 is my favorite by far. Keep in mind that while not always easy to do, the idea is to get the cutter centered around the spotweld and only cut through the first layer of metal then break the weld with a spotweld chisel (available from Eastwood).

The trick to filling a hole is a big chunk of copper as a backer to absorb the heat. Here's a real handy tool for filling holes - got it off Amazon. It's magnetic to clamp onto the panel and the screw adjusts the copper to the hole.

I have used a lot of Blair cutters over the years. Get real long life, but plenty of cutting fluid and very low speeds. Speed kills the cutters.

same here. i have used the same blair rotobroch cutter for 3 YEARS. i am a autobody tech and have used it on 1000+ spot welds in that time.
i first drill a 1/8" pilot hole. then dip the cutter in oil and drill at about 500 rpm in my 40yo 1/2" CP air drill.
the key is SOW and use 2 hands on the drill. if the bit grabs it and twist the drill and shatter it in 3 millisecond.

and you can't use it on high strength steel.

as far as copper backing.
i like to use these. i think i have about 6 of them

copper welding vice grips

It is amazing what heat water cooled copper can resist.

From 1990 to 1996 I used a “vacuum melt arc furnace” to make compounds and alloys.

This type of device is kind of a cross between
an “air arc”,
a vacuum chamber,
and a 4” thick pure copper plate with water cooling holes drilled through it.

I could strike a 100 amp electric arc through a tungsten electrode hitting the copper base plate and melt nearly anything.
The vacuum prevented oxygen from burning the copper and the water cooling prevented the copper from melting.

I could react pure chemicals together to make alloys,
then use the electric arc to “push” the molten blob into various shaped molds in the copper,
where it would cool into an ingot that in several shapes: buttons, rods, squares, hemispheres, triangles, etc.

I reacted all the “rare earth metals” with pure carbon to make rare earth carbides,
and later I reacted rare earths with the expensive “carbon-13” isotope.

On special request in 1991 I replaced the normal tungsten rod with a carbon rod and created “soot” with the electric arc that we scraped off the copper and dropped into benzene.
The benzene turned red,
indicating “buckyballs” had been created.

Amazing what water cooled copper can resist.

I started with the cheap 3/8 ones and broke a bunch of the heads. Figured out I was going too fast. Got a Blair 3/8" and it works better, but you need a good center punch or partial drilled hole to keep it centered.
Then started using this 5/16" one that is like an end mill with a center tit. Nearly self centering, requires a little more pressure but cuts really well and I appreciate the smaller hole on the panels I will reuse. I use all of them, depending on what I am removing.
https://www.amazon.com/gp/product/B005TY5EPI/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

The Lisle spot weld chisel is worth the investment too
https://www.amazon.com/gp/product/B000BQXBJK/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&th=1

If I will be tossing the outer panel, I just use a belt grinder. Less tedious and doesn't leave a pilot hole in the other panel. https://www.ebay.com/sch/i.html?_nkw=dynabrade%20belt%20sander%2018100&norover=1&mkevt=1&mkrid=711-34002-13078-0&mkcid=2&keyword=dynabrade%20belt%20sander%2018100&crlp=_5087&MT_ID=&geo_id=&rlsatarget=kwd-76965991551231:loc-190&adpos=&device=c&mktype=&loc=85042&poi=&abcId=&cmpgn=395412448&sitelnk=&adgroupid=1231453229708374&network=o&matchtype=e&msclkid=3f5a3216c7bb13e5b795538af6dc0c83

https://www.ebay.com/itm/392266544522?epid=5030361753&hash=item5b54e8758a:g:KOsAAOSw3tBdMKRc

belt grinders are the best if you are scrapping the panel. WAY faster than drilling out spot welds.
they are really great when sanding down the weld nugget. i like them better than using a grinding disc to do that.
they really shine when you have to remove a ultra high strength steel panel. regular cobalt bits won't even put a tiny divot before going dull. they do make special bits for this. they are $$$$ though. sanding belts are cheaper.

https://www.ebay.com/itm/392266544522?

https://www.ebay.com/sch/i.html?

HF sells mini belt sanders also.

electric one....HF electric belt sander

i thought they also had a air powered one also. i can't find with a quick website search.