Sounds like you are working with some dirty material. Just use some solvent (break cleaner works) and clean the material. You shouldn't have to wire brush or clean the filler unless its been laying of the floor getting stepped on and that times to much time. And btw I would recommend putting a beve...

I am butt welding 1/8 X 6/6 aluminum tubes. The welds looked perfect until I ground the weld off. It had gas bubbles down the length of the seam. why? By the way, I'm not beveling the aluminum, I'm using 20 pounds of argon with a 1/8 inch tungsten.