I'm in the middle of a car restoration project - a 68 california special mustang.

Metal must be clean, no rust, no oil, no wind blowing the shielding gas away.

About 20 cfm on the gas works the best for me.

Danny

What's the problem?....are you trying to burn through the to piece to weld to the bottom? Are you grounded? Tac welding or continuous?....more info needed to tell you what's going on...

Pretty much the same for me.. I run 25-30 for gas pressure.


OP can you take pic's of the welds?

You should only be tack welding the floor panel back in if it is a full replacement panel. You'll need to drill 1/8"-3/16" holes around the perimeter of the new panel to tack through, just like a factory spot welded panel. Make sure the replacement panel fits tight, then tack it in one spot, then jump around the perimeter of the panel.



Try practicing on some scrap for tacking through the hole. Starts and stops are the weakest point of any weld. . .and a tack weld is both! You may need to turn the heat up a bit to make solid tack welds. Unless you are doing some strange fitment in an odd area, you shouldn't need to stitch weld the panel. If you feel the need to do so, such as where the panel runs on top of the subframe, make sure to bounce around a lot to keep distortion to a minimum.

Gas should be set to around 30.  Describe your welds, if they're full of porosity (little pin holes) then you need more gas and to clean the surface better, it's contaminated.  What gauge metal are you welding? If it's 18 or 20 ga I would switch to .025 wire.  If you've already cut the pieces to fit as a lap joint (slight overlap) then drilling and plug welding will be out of the question.  You're going to need to stitch it.  To avoid warping the metal you're going to have to jump around.  Start with a small bead, no more than an inch in length 1/2 inch would probably be the best in the top left corner of the panel for instance.  then go to the bottom right corner, then bottom left corner, then top right corner.  Then keep skipping around like that to avoid putting too much heat in the metal.  Make sure you burn into the beginning and ends of the previous welds, but don't blow through.  keep practicing on scrap of the same thickness of your work panels till you get it running right.  Tweek your wire speed and amperage till she's running right.  Don't "microwave" it, that's running too hot with not enough wire.  Too cold and your beads will "pile up".  It will take some adjusting but you'll get it.   Use acetone or denatured alcohol to clean both pieces of steel, and if they're coated use a small angle grinder with a sanding disk on it to buzz through any rust or other coatings.


eta:  pics of the welds too...

I'll have to get some pics of my practice welds tomorrow or Sat. Everything is at my shop about 5 miles from my house.

The pin holes are your first sign of a contaminated weld.  Try turning your gas up like I mentioned, and your heat down a touch.