I have a few sizes of rosin core solder I use for electrical work, from the smallest I could get to a size proper for soldering 12 guage stranded wire. I use copper braid for de-soldering work. I prefer an iron for more delicate work, and a gun for heavier stuff. I always wipe off any old solder with a wet rag after the gun/iron first heats up, and re-tin it before proceeding. For technique, I make sure gun/iron is fully heated, then touch it to one side of the wire. After I think it's hot enough, I test by touching the end of the solder to the other side of the wire, and it will either melt and flow readily and I will proceed, or I get in idea how much more it needs. When it's hot enough, it will melt and flow very quickly, so you have to be ready to feed it if necessary, or remove pull the solder away. There's some feel and judgment to it. With some experience it will become easy.