flux advice

[algorithm]

Brilliant, thank you both, very informative. 

[IanB]

If you try dehydrating it further, it doesn't let the water go easily, and HCl comes off as well, leaving some mix of zinc chloride and hydroxide/oxide.

Apparently this decomposition also happens under the heat of soldering and the liberated HCl helps to dissolve the surface oxide layer on the item being soldered.

[KWuensch]

It depends on what type of solder or flux you were using for the failed projects. If you are using a no clean wire solder or flux like these:
http://www.gotopac.com/AIM_AF_NC264_5_1GAL_p/af-nc264-5-1gal.htm
http://www.gotopac.com/AIM_AW6337_010NC209_p/aw6337-010nc209.htm
You should be able to leave the left over flux on there since those are non-conductive. However, if you are not using a no clean solder or flux, you should be removing the excess flux when you are finished. You should be able to clean it off with IPA once you have finished soldering.