Why do my flux core welds look like burnt popcorn?

That problem with the tubing is super common when moving from flat plate to box sections because of how the heat behaves inside the corner. When you weld thin tubing, the material stays hot for a longer time before cooling, and the inside walls are prone to heat-affected zone issues that just don't show up on flat scrap. The biggest difference is that in the corner, you're dealing with restricted heat flow, and if you're not bridging the gap properly, the puddle just sits in the corner and cools or rolls off, leading to that messy look. It's less about the heat setting and more about how you start and finish the weld on that tight radius. Also, you're right that your fit up is a massive factor here. If you've got huge gaps and overlapping pieces, you're not welding a joint; you're trying to bridge a structural gap. Start by making sure every single piece is fully supported and stable while you're running the bead. Use tack welds extensively, but don't just tack the ends. Run short, small tack welds on the inside corner where the pieces meet to hold the alignment before you commit to the full pass. This ensures your joint stays square and stable during the heating process.

To address the holes and the lumpy appearance, you've likely hit a combination of high heat input and poor travel speed over the puddle. Even if you think you turned the heat down, if you're moving the torch too slowly over that gap, you're dumping too much energy into a small area, wich causes boil-holes. For those thin walls, it's often better to do very short, fast runs rather than one long, slow bead. If you have a gap, you need to fill it incrementally. Fill the gap with small, precise root passes first, then layer your fill passes. Don't try to force a large gap closed in one shot. The angle of your torch might also need adjustment for the corner. Sometimes leaning the torch more into the joint helps push the puddle into the tight space instead of letting it run along the outside surface. Keep the technique tight, and you'll start seeing much more solid results.

Even after fixing the fit-up, I saw weld failure on a few joints until I made sure everything was spotless. The biggest change for me wasn't the wire speed, but cleaning the metal completley before I started. I used a simple wire brush to knock off all the oxidation and grime from the inside of the tube. That initial preparation, which sounds minor, is what made the difference in achieving a solid fusion. On the thin walls, any tiny bit of contamination acts like a crack and causes those little holes. If you aren't sure the surface is totally clean, just give it another swipe.

You gotta remember that flux core is a rush, kid. When I was running these kinds of thin pieces, I didn't spend all day looking at charts. If you're blowing holes, it usally means the heat is spread too thin across the material. Try focusing less on the travel speed and more on the actual voltage setting. Sometimes dropping the voltage a notch or two helps keep the puddle tight and contained right in the groove without overheating the surrounding wall. It’s all about controlling that heat input properly.

I am really new to this stuff too, and I was working on something with thicker square tubing for a frame and running into similar issues. It seems like even with better fit up, getting a nice smooth bead is a nightmare if you are moving too fast. I sometimes wonder if the problem is related to the actual metal itself, like if I am accidentally getting a lot of slag underneath the flux that I just dont see. I tried making sure the flux was fully melted before I started moving, but I still just get these rough spots. Is there a trick to making sure the metal stays hot enough to flow right without burning through it?

I'm skeptical about the idea that just doing very short, fast runs will magically fix all those holes. While speed helps manage heat input, if you rush a joint like that, you're just barely fusing the metal togather. You might get a clean look, but you defintely won't have the root penetration needed for a load-bearing structure. I've found that when dealing with thin square tubing, the goal isn't just speed, it's controlling the arc length and ensuring you're fully welding the toe of the weld onto the base material for sound integrity.