it sounds like you're still not getting enough shielding down in that corner. the geometry of the joint you're trying to weld makes it hard to get the cup in close to the joint. there are a couple things you can try:
- clean your parts with a solvent like acetone or lacquer thinner. you may have some residual grease. also, make sure there is no mill/fire scale near your joint. you're grinding a bevel, so you shouldn't have any there, but grid back a bit past your bevel and also the spot where it meets the band. cleaner is definitely better.
- make a tape and cardboard dam around the part on your welding table. argon is heavier than air, so it will fill up the dam and push the oxygen out giving you a nice shielding blanket. if you're going to go this route, you need to be inside with NO draft, or you shouldn't wase the time. air movement will just suck out the argon from your dam. speaking of, if you've got significant air movement where you're welding, you're going to lose your shielding gas or get air sucked into your shielding gas which will create porosity.
- buy a gas lens collet body and a #8 or so gas lens cup. you can turn the flow up a bit and extend the electrode out farther so you can see what you're doing and still fully shield your joint. I think i paid $4.50 for my collet body and $2 for the cup from mcmaster carr(not the best place for welding stuff, but fast shipping and and good prices for most stuff.
- buy a plastic sandblast cabinet from harbor freight and plumb argon in. since you're welding relatively small objects, this may be the best long term option. i think the large cabinet is $150.
- practice!
some of that porosity may still be technique. try doing the weld in a few passes and wire brush between them. keep your arc nice and tight (distance between tungsten and work). try different hand grips, differnet head positions. get comfortable. you'd be surprised how much better you weld when you can see exactly what you're doing and you're comfortable, even if you're welding upsidedown under a car.
- keep your tungsten clean and pointed. if you dip it in the puddle or blob the rod on it, stop and clean it off. it's ok to keep going if you're practicing your torch hold, arc length, ect. but for finished work, you need to keep that sucker clean! don't be tempted to just finish that last little bit if you've got a big nugget on the electrode.
-if you do notice that you're getting porosity, stop and grind it out before you continue. rarely, you can burn it out, but generally that just moves the gas bubbles around instead of getting rid of them.
if you're still having trouble, post pics, it makes it easier to figure out whats going on
good luck!