Depends on what your requirements for penetration are. Also, are you back gouging? Some Fit's that I have welded on .065 stainless proccess piping need to be touching. You then watch for the argon to float up from your purge and weld it "from the in side out". Other fits on heavy wall material I.E. 1" thick or better I have gone with as much as a half inch gap, then back gouged out the porosity and filled it up. One of the proper ways to weld schedule 40 pipe for high pressure applications, is to gap the pipe 3/32" and then fill it with a 1/8" fast freeze rod. You can actually see and hear proper root penetration. This ensures 100% penetration. I guess what I am trying to say is that there is the proper way to do it and the right way to do it. What ever the application is will dictate the procedure. If you are doing hobby work and don't have to worry about your weld breaking from the inside out, a tight fit and less penetration is faster and easier. But if your welds are going to withstand high pressures you better make sure you have 100% penetration.

the only time i'd want a gap is if i was doing a butt weld on something...and like you said, it depends on the thickness of the material. fillet welds and corner welds need no gap...the tighter the better...unless ofcourse you're talking about 1 1/2" or thicker.

i remember you mentioning a butt weld on your trailer...if it was 1/4" thick, i would have also V-grooved it, fit them together so they're touching, weld on one side, and then like stitch said, go on the other side of the weld and grind it down until you see clean steel (back gouging), then weld that side. to me, this is the easiest way to ensure a good strong weld w/o messing around w/ an open root (a gap).

but then again, sometimes it's too hard to back gouge, like with an H beam, or a piece of C channel. but there are ways you can reinforce a joint too...weld it up, grind it all down flat w/ the rest of the piece, and splice another piece of steel over the joint and weld that all the way around.

A little gap in your joints, especially when you’re using GMAW-SC, is definitely a plus! That’s how it’s taught and there’s a reason for it. After you’ve completed the weld, a 1/16” gap will have closed and you’ll have a tight fit due to the weld cooling. Whether you’re a hobbits or not, the fact that GMAW has a low penetrating arc due to it’s characteristic low heat input. If you want to help insure yourself a fully penetrated and fused weld (to the bottom of the joint) use a little gap and burn those little edges off your corner joint.

I am certainly no expert although for the last 4 years all I do 54 hours a week is butt welds on steel joist top chords on material from .109 thick up to 1/2" and about 10 sizes in between. I was taught (doesnt mean its right!) make your gap 1/2 of the material thickness. I have done numerous tests on angle steel I have welded by bending it until it either bends into a "U" or the weld breaks. You would be very surprized at how little penetration you get with no gap, and I am using a very large commercial production machine (MIG) that will put out more heat that I could ever think of using. Just my thoughts.