DVST8R: Thanks for the tips. I was actually shocked how well the flux-core rod worked, which tells me a lot of my previous problems must have been caused by inadequate and/or inconsistent fluxing. This rod is amazing... on that first test I mentioned, I only prepped one area very lightly and quickly by hand scrubbing the oxides off the surface with a brush and I left another area totally original, old cast oxidized. I did apply some supplemental flux across the whole surface I was welding on for good measure. The prepped area definitely welded a lot better, and prepping it more could only have helped. But I was shocked I was even able to weld the completely unprepped area together! Wasn't that easy, but I found like welding stainless, I was able to "scratch" the surface to help get the the metals to flow and join. I will definitely spend a lot more time prepping on any real welding project I do, this was just an experiment to see how the new rod worked. Getting good fusion was so much easier with the right amount of flux being where it needs to be. When I built up some filler material on the surface, I no longer had problems getting it to "sink in" and flow out.
My latest aluminum welding problem is that sometimes when the flux gets thick right where I'm welding, it glows so brightly I can't tell where the molten puddle is! On my test this only happened once when I ran a bead towards another area I already welded, but I had to stop when it happened. They make special welding lenses to block out this "sodium flare", which is I think may be the last piece of new equipment I'll need before being ready to tackle the real projects.
Oh, my test piece of aluminum, the weld really isn't too pretty. Heat management is really tricky with this stuff, so I didn't run the prettiest bead. It turned out to be a much more difficult weld than I planned because it was one small thin piece joining to a thick piece. I didn't melt through anywhere though (right at the beginning, I was very close though!) I'll practice up a little more.

PS - I might have to try those chemical cleaners you mentioned.
moTthediesel: VNT-15 and a long-runner intake... :shock: going for a little bit of torque, eh?

I was able to get an unmodified A2 manifold to fit the VNT-15, no manifold spacer required. Pics are in the VNT 1.6 diesel thread. I removed one of the studs from the EGR flange and replaced it with a countersunk allenhead cap screw that I rounded the shoulder partially off of to make additional clearance, but it all fit bolted up with the VNT-15 and the unmodified A2 manifold to a 1.6 liter head. A specially contoured EGR block off plate (which I made in stainless) was a key ingredient. Even made an intake manifold support brace. I didn't end up using any of it. I held on to the pieces and they aren't doing much sitting in my garage. You don't want to know how many hours I have into them. If you could use them for your project (also could part with the A2 intake, which you might want since it's been tested to fit, and also has the mounting flange to bolt up the brace), send me an IM.
Also, although I never tried test fitting an A1 intake manifold to a VNT-15, it might fit easier than the A2 so might be worth considering. It's got shorter runners though.