Lap welding techniques

[jaredts]

I should probably start a project on my truck, cause that's what it is!  '83 Silverado, bad rust in some spots, but not bad overall.  I'm doing the floor right now and just finished struggling on the lap weld on my inner rocker.  I know part of my problem is that I'm using .035 wire instead of .023 (this is a mig).  I would consider getting the smaller wire, but wondered how much of this is technique--especially since I have made some pretty good welds.  Problem is that the technique that seems to be working the best is to turn the volts and wire feed up kind of high and move the gun extremely fast (too fast to control and duplicate).  Anything else seems to burn away the top piece when I lap weld.  Very quick bursts with small circles somewhat works too, but burning through the top piece is so easy.  I'm into my floor now and its not going too bad, but any help would be appreciated.  I guess in short it seems like I can't move the gun fast enough to not burn through the top piece at any volts/feed.

[Jim Rockford]

But welds are better,lap welds invite more rust between the panels. Butt welding takes a little more time and skill, but provides a much better finished product.

this whole quarter was hacked off,

With some skill the new one was welded on and took very little filler.

[micahadams]

Im assuming you have shielding gas like c25? The .023 wire will help. Keep the voltage on the lowest setting and stitch weld it. Also some people believe a pull technique introdues less heat

[jaredts]

Butt welding is out when it comes to the floor (if for no other reason than I'm partially finished).  I did a few butt welds lengthening and modifying my inner rocker and they turned out great.  Lots of plug welds with good success, too.  Its just something about the lap welding that's difficult to do with .035 wire or I'm not doing something right.  Could anyone describe the preferred motion?  Lots of little circles, straight line up the seam, zig-zag??? Definitely use the lowest setting?  I know that may seem obvious but that didn't really seem to make things better?  I am using shielding gas by the way (c02/argon).

[Donut]

Start your puddle on the base and let it build to consume the top.  Keeping your gaps TIGHT helps a bunch too.
Set your heat and wire speed on some scrap that's close to the same gauge as your welding.  Make sure it's as clean as possible, don't grind out to much metal, but don't try to go over rust, paint, grease etc.
Go small, not much more than a series of tacks, and jump around to let the metal cool.  If somethings not right, STOP, fix it.

I started doing my floors with .035 flux core, but had much better luck using .030 with shielding gas.

[Jim Rockford]

Problem with lap welding on floorpans  and everything in general, is you create a place between the panels for moisture to get into and create rust, and Man I hate rust.