Are you Just rewelding the foot on or welding the tubes together? If welding the tubes together I always make a small sleeve to go between the tubes. That gives your joint a good weld backing.
As KL says drill a small hole in the bottom of the foot to let the gases out. You will be surprised the differance that will make. Sometimes I get lazy and don't and I always regret it. Also if you get a stubborn porosity hole you can try a little stainless rod for that spot on a non critical joint. bye