Not knowing your soldering experience:Solder not wetting at all.
Flux is just a chemical way of removing tarnish from the parts being soldered. Most people do not have rosin based flux in their toolbox so using physical methods to brighten up the copper or tinned component leads will let most people get their part soldered today with the tools at hand instead of running out to Radio Shack to get s pot of flux. Oh wait, have to wait a couple days for amazon to deliver it - no more radio shack.If the parts are obviously tarnished, clean them first - but that is quite a rare thing to have to do.
Er... It's in the flux-cored solder, or they have a serious problem trying to do any electronic soldering!Most people do not have rosin based flux in their toolbox
Oh, I'm not sure. The solder I use at home is labeled as "rosin core", but maybe the name just carried through. I don't recall off the top of my head exactly what type of flux is in it.Do they even make rosin cored solder any more?, obviously all electronics solder is flux cored, but wasn't rosin removed on health grounds?.