Anyone use one for doing mods? How well do they work ?

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Not pretty but as long as you know what your doing, it will get the job done. Just have a lot of splatter to deal with.

I got a stick welder and a flux core I'm pretty good with both but was just wondering.

I use one, use nozzle dip and splatter spray . Works good enough for the mods. I do mainly welding the j's shut.

I have a Hobart 125 EZ flux core welder. I do all my trap mods with it. Base plates, bubble tipped jaws, laminations, etc. I like it and can make some pretty decent beads with it. A lot of spatter and smoke , but makes good welds if you do your part.

Its what I use on my traps there is some clean up to get rid of the splatter. I use a wire wheel or my dremel with a sanding drum to clean things up.
I'm using an old harbor freight 90 amp flux core that I converted to DC output like the lincoln's and hobart's output. Smoother welding and way less splatter/bb's when it AC output.
Justin