The type of solder you use matters less than making sure it's cleaned and heated properly. I've used both silver and rosin core with satisfactory results. I use a weller portasol. It's a little butane soldering iron/torch. It's a little on the pricey side, but once you get one, you'll pitch all of those bulky irons that tie you to the wall and are never hot enough when you need them to be. It also works really nice for removing those sections of monokote you would normally use a hobby knife for (middle of the horiz stab/ bottom of the verticle stab).