An angle grinder, small, one hand type, is often used more than the welder, when welding. Cleans up the metal for welding, cleans off splatter, sharp corners and excessive bead. Generally and pratically the best, handy hand tool, a welder needs to be competant with, along with a good set/variety of locking pliers, c-clamps and such.
Technique is something that´s learned. General technique for rod welding is to point the rod in the oppostite direction you are welding. This piles up the flux behind the work. Also in most constructions, a series of 1 inch or so welds, works better than one continious weld. Kind of like a long spot weld. A continious weld is often unnecessary and the extra heat just causes problems. Think of the weld like any fastener, a bolt, a rivet or whatever. Make it twice as much mass, as a bolt, that would be needed as a fastener in the same situation. Any more is often redundant and doesn´t add to strength. And is probably twice as strong as it needs to be anyway.
Welding inside corners, has often been a problem. The heat reflects and the hot gasses reflect off of the sides of a right angle, which causes bubbles, splatters and excessive flux buildup. Very clean metal, smaller rod and a slightly higher heat often helps. Much of welding is technique and feel, which you´ll pick up with experience.