Screw that crap. You need a separate can of it. Available @ radio shack. I forget what brand we have at work but I use it liberally on switches and what not when soldering in the cockpit. Easy-peasy.
I just live here.
THis...... If your just making a western union connection with two wires than the flux core works fine, but for anything with detail get some solid wire solder and a can of flux.You sound like you're good at it. I might just pay the local shop the $30 to wire everything up (pickups/controls in my guitar)
I know when I've met my match and am not ashamed to bow out.
Without asking anything I will take a stab and say you are probably using too hot of a tip, and not letting teh heat transfer into the connection before adding your solder.
Being your working on a guitar You may find tht it will be easier to "sweat" the parts together.... IE solder the end of your wire, solder the place you wish to connect it, hold them together heat just enough for them to melt together and BAM your done.
Also for small electronics get some 50/50 instead of the 60/40.... Melts a little easier, and is easier to control it.