Step 5: Preparation before welding
A friend brought me a bicycle rack for the bumper and trunk lid of their automobile. A 1/2 inch rod had broken off near the end of the threaded portion. She and her husband thought I would simply run a weld bead around the break. They were surprised when I made a deep "V" across the top and bottom of the joint. Then I made a root weld at the thin leading edge of each piece to join them together. See the red oval in the graphic. The root weld is often done with a thinner welding rod or more current or both to guarantee excellent penetration. I waited until the joint had cooled and chipped away the slag. Grind a little on the back side, too, to avoid any slag being trapped under a weld. Such slag inclusions would weaken the weld. I made a weld on both the top and the bottom of the root weld to keep stresses in the metal equalized. See the yellow and blue in the graphic. Clean away the slag after the welds cool and weld on both sides again. I repeated this process until the "V"s were full. Instead of a few minutes, this project took more than an hour to complete. You can see a real life example of this at this Instructable. The same Instructable is also linked in step 7 in regard to preheating.
An old adage is to weld a little and cool a lot. After welding, pounding on the welded joint with a chipping hammer reduces stresses in the weld.
There is also the matter of duty cycle. Every welder has a duty cycle. That means it will overheat and stop welding if used continuously. The duty cycle is often 20/80. That means after welding for 2 minutes (20 percent) you must let the welder cool for 8 minutes (80 percent). Often a weld can be completed in less than 2 minutes and you will need more than 8 minutes to get ready for the next weld, anyway, so it all works out.