In a complex piece you must sometimes solder several elements consecutively in the same area. Some people use different types of solder – hard, medium and easy – starting with the hard solder that takes longer to flow and then moving on to medium and finally easy as the work progresses. If you only need to solder three or four times, that could work, but if you need 20 or more soldering points, it won’t do you any good.
The alternative is to protect previous soldered areas by preventing the solder from flowing again when you reheat the piece. For this to happen you need to get the soldered area “dirty” because solder will only flow if the metal is clean.
The traditional method is to make a paste with rouge powder or any other polishing compound mixed with water. It works but you have to make it every time, so it’s not very practical. If you want to try it, though, this site explains how.
My favourite method, that I find a lot easier, is what I learned in jewellery class: use white correction fluid over the soldered area. The liquid kind, with the brush, is the most practical. I also use the pens. Apply it over cold metal, allow it to dry (or it will catch fire) and then solder the new element. Just be careful not to let the correction fluid touch the area you are soldering because it will also prevent the new solder from flowing correctly.
White correction fluid to protect previous soldered spots
Correction fluid turns black but protected solder doesn’t flow again.
Pickle and a brass brush will remove all traces of the correction fluid and you can reapply as many times as you need. Just don’t reapply over hot metal or you will burn the brush.
Important safety concern: the vapours produced by the solvent based correction fluid shouldn’t be inhaled. During both application and soldering, a respirator mask or good fume hood should be used.