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What you use will largely be determined by which panel you are saving, what access you have to either side, and how well you and the tool work in harmony.

If you are facing the piece you wish to save, and the lower panel is essentially a throwaway, then simply use a drill bit the same size you would select for plug welds, and drill out the spot weld. When you place the new panel beneath and clamp it up, the hole you drilled in the top panel now serves as your plug weld hole.

If you are saving the lower panel, and throwing away the top, you can use whatever works best for you. There are many ways to accomplish the same thing, but if you are not proficient in using one method, try another. I think each method has it's faults, so pick the one that best suits the area you are working.

The holesaw type cutter typically cuts like any other holesaw does, and once the teeth start to cut a "channel", it is difficult to see how far along you are progressing. At the point the cutter reaches the second layer, which is where you would want to stop, if there were a bit of rust between the two panels and your cutter had enough speed, you would have a visual indicator in a wisp of "rust smoke" that is seen coming around the cutter. It is here that, even though some moderate speed is needed to produce this indicator, light pressure is also needed (better classified as "restraint") so that you don't go through too far and damage the second panel. In my case, I found myself going through too far, and would either need to repair the deep channel I just cut in the second layer, or would have to weld in a circle to repair the gaping hole I just left. Needless to say, I no longer use this method, and gave the cutters I did have to someone else that hopefully is having better luck with it than I did.

Some of the cutters have a spring loaded center punch, much like a machinists roto-bore. Even with an intial center punch used in the middle of the spot weld, These cutters have the misfortune of slipping off center, and many people will simply drill a 1/8" diameter hole, either partially or all the way thru, to prevent the cutter from walking about. I'll stop here and offer a generalized thought. If you have difficulty filling an 1/8" hole in a piece of sheet metal with your welder, you will likely have problems with the pilot drill method, and perhaps should try one of the other methods.

I think the Wivco cutters will work better than the hole saw type, in that they mimick an end mill, so the cutter is relatively flat on the bottom. This should give a less aggressive cut, a plus for people like me who may have a problem of leaning too hard on the cutter. The open flute design will also allow you to better see what is going on than the holesaw type, which obscures everything. It does use the pilot bit, so if that is not an issue (see above paragraph), then this is a good choice.

http://www.tonsoftools.com/images/Wvth25000.jpg

The blair cutter is available in either the spring loaded version or the pilot bit version, I think these are likely a more aggresive cut than the wivco, especially since the cutting surface is extremely narrow, so it may be more likely to pose the cut through problems I described initially.

http://cdn3.volusion.com/uo9gz.acro2...jpg?1470814969

A rounded burr grinder is also a good method which should somewhat limit the damage to the (throwaway) panel to just slightly larger than that of a spot weld cutter. The downside is that these also come with little tiny slivers of metal that are a pain when you get them in your skin, so it would be advantageous to address these with a vacuum cleaner/foxtail and dust pan on occasion to keep the issue at bay. A pair of work gloves come in handy as well. Keep some duct tape handy to pull out the slivers that sneak by.

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The last method I'll discuss is the one that I use because I don't play nice with the holesaw type. I tend to inflict enough damage that I'll need to fix hole saw size holes through the save panel, or at least deep grooves. I use a 3" cutoff wheel with a 1/16" thick cutoff disc and use the tool to grind away the spot weld. I find for myself, it offers a less obstructed view of any of the methods listed, and with the proper speed (fast), will give you an indicator in the discoloration of the top layer (blue or darkened) usually before you have even broken into the second layer. Basically the metal is heating up and as it starts to get thin, it heats up more quickly and shows this via a color change. The color change back to bright silver will indicate you have reached the second layer, and act as a guide where to not grind anymore (the bright area) and where to grind, the blue/dark circle surround it. The disadvantages with this method, are the top panel is basically useless now, you will need good eye protection (moreso than the holesaw type cutters), and due to the grinding particulate, will need to use a respirator/mask to prevent you from hocking up black globs shortly afterward. I usually get a 3M or equivalent paint respirator, as the typical dust masks only serve to fog up your safety glasses, and as is evident upon the removal of the dust mask, they don't work all that well. A paint style respirator exhales to the sides, away from your safety glasses, and typically conforms to your face much better.

Grind pattern visible:

http://i5.photobucket.com/albums/y16...n/HPIM5779.jpg

That should give a brief view of both the good, the bad, the ugly with each method, now it will be up to you to figure out which one works best for you and best for the situation at hand