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I use flux core all the time, just with a lower heat setting than I do with gas, i don't know why for sure but it just works better. But I don't weld beads on body panels, its just more time consuming with flux core like he said..but its fine.
i've never had a problem with using flux core..but i also grind all my welds smooth. Also at one of the workshops i went to, they said that self-etching primer will eliminate any problems with adhesion..even if there is flux still there or not.
Depends really, if it's something like a floorpan, I want it to look good and also be strong, but I'm not as worried about a bit of an overlap as I would be with a rust repair in a more noticeable area on the exterior of the car. For something like that I'll be sure to cut and fit everything nice and snug, then stitch it in slow to keep the metal from warping.
I generally butt weld, but what I do is bevel both panels so one sorta lies on top of the other, then weld through both. This allows a slightly higher heat setting which will give you a better bead overall with most consumer wire welders.
Since body panels are thin they tend to warp with heat, so you want to "stich" one area, then move away from that and stitch another. Keep doing that around the perimeter of the panel, then go back and start off at the first bead, and continue a bit, and keep doing that until the bead is completely around the patch panel. This way you're not doing one huge bead all the way around which will result in blow-through and warping.
Another way to control heat and blow through is to back the two panels up with copper. A steel weld will not stick to copper.
I have made several tools for welding using lengths of copper plumbing pipe. I make a 2-3 inch slit parallel to the lenght of the pipe and a cut perpendicular to the first to allow the pipe to be opened and flattened. You can either leave a piece for a handle or just use a flattened sheet. I have also used sheet metal screws to hold flat cooper sheets and keep the panels in alignment.
When you weld your panels warpage can be better controlled if you make a few short tacks, grind them flush and straighten any warpage with a hammer and dolly before adding more tacks between the original tacks.
If you go slow and straighten the panels as you weld and not after your final product will need less filler.
Like has been said before..depends on the area. Like the cab corners for my 54 I used a butt weld because its seen and needs to be very clean and flush. But the floor pans on a 65 Chevelle that is in the shop now I used a lap weld with holes punched in it so I could do spot welds in there and then go back and weld along the seam.
I have heard of using the copper backing(either tube or sheet) to act as a heat sink but have not tried it as I usually cannot get behind my weld very easily.
I do always use a hammer and dolly whenever I can to make sure the panel is as straight as I can get it before using filler. Just make sure if you go to do that you have a good set of spoons, dollys, and hammers.
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