It is also messy. I have a deep metal baking pan that I use for fluxing. I keep my flux in coffee cans inside the pan and use a long handle spoon that I made to apply it. Some folks like using a salt shaker. The second welding heat, you start at the handle end and go in the opposite direction then the first heat.
Most flaws occur at the ends of the billet.
Unfortunately, these get worked into the middle of the piece during the folding and welding. If you are in doubt about a weld taking, forge lightly on the edges and see if it opens up. If it does, reflux while it slightly open and reweld. Periodically while I am welding up billet I allow it to cool for what I call a grind and check. I grind the edges of the billet looking for flaws and inclusions.