Well I use resin cored solder as well as using an externally applied flux, copper will oxidize very quickly at the temprature that solder will melt at, even perfectly clean copper will have oxidized so much by the time it's up to temp that flux will still be needed.
I wouldn't worry too much about messing it up, that's the bonus of using lead/tin based solder, if you arse it all up simply heat the block back up and touch up the solder work, or take the block apart entirely, clean it up and try again.
Some of my blocks have been soldered, de-soldered then re-soldered many times.
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feel free to icq/msn me, I'm always willing to toss around ideas.
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