Some of you know how to do this, some of you don't, i am going to show you the easy way to do this, especially if you need or want to save both pieces that are welded together.
Typically if you have one piece you need to save and the other you are discarding because of rust, you drill and separate from the side you are discarding, or grind down the spot weld with a grinder on the side you are discarding, and use a chisel to separate the pieces discarding the bad metal.
In my case i am separating a rust free transition pan from solid rear frame rails and need to keep both pieces. Logic dictates drilling the spot welds on the transition pan side, and then there's already holes in the pan to rosette weld it into place into the car its going into. This leaves the frame rails hole free, or with divots from a drill that can be filled with weld and ground flat prior to being installed in a different car.
The start of the spot weld removal after determining the side you are drilling on is to clean up around the weld to find its exact center, then using an 1/8" drill bit spot drill the center of the spot weld, but not all the way through. This will help center a larger bit. I removed the floor pan to frame rail spot welds. These are fairly big and require a 3/8" drill bit to drill out. Other body spot welds may not be as beefy and smaller bits can be used to drill them out. I determined these at 3/8" because the spot was about the size, or slightly smaller than a 3/8" bit shank diameter.
Here's first 4 pix in chronological order. Notice the drills don't go all the way through the frame rail at the spot weld, only far enough to remove or severely weaken it.
The next step is separating the pieces once you have the spot welds drilled. I have some special made heavy duty spot weld chisels i made from .050" stainless steel sheet stock with sharp forward edges, and handles that can be smacked with a hammer. These are what i use to get between the sheets that are spot welded together. The long one is about 2.5 foot long and about 3" wide. This one is perfect for running up the frame rails after drilling welds to pop em free. The .050" stainless is thick enough with a sharp edge to cut, and thin enough to be flexible to follow the frame rail curvature when tapped in with a hammer. Pic #5 you can see it tapped in all the way to the handle and the welds are popped free.
The smaller chisel has seen better days. I am in the process of making a new handle and new longer chisel. The handle on the smaller chisel is made from 2 pieces of 1/4" aluminum plate bolted together. I made this one 20 years ago. It used to have 6 fasteners holding the handle together back then. The blade was about 3" longer then as well. The longer chisel i folded over the stainless multiple times over itself, then beat it flat on an anvil. used monel steel aircraft rivets in it to keep it from separating.
A word of caution. When using the chisels to slice thru the weakened spot welds, if the chisel stops in the middle of a drilled weld, if you try to keep driving it in, you could tear the metal at the drilled hole because some of the metal is still attaching both pieces well enough. If this happens where the chisel stops, try to run it in 180° from the other direction, or at least 90° from the sides and drive it back in. 9 times out of 10 you can break the rest of the weld loose this way.
To give you an idea how easy it is to do this, i removed 2 frame rails, a rear cross member, and a shock cross member from the transition pan in about 3.5 hours. The last rail, the RH side took me only about 1 hour to remove. No damage to the rails, , transition pan, or cross members. The pan will clean up nicely, and already has holes in it to weld it into the car its going in.