Well if you need to control your flux you need to mix it to a thicker paste and put less onto your piece (stops it spreading)
Soldering a seam or a bezel (when making a box or locket) I always warm the piece before painting on my borax (paste)
this burns off and eliminates any excess grease and helps the flux to travel into and through any seams you're about to solder.
No matter what flux you use you can contain your solder by first painting on jewellers rouge (solder won't run onto it)
Carlton
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