Rosin core solder is what is generally available in places like Radio Shack and is the way most solder is made that IS NOT for plumbing. The flux is actually in the core of the solder.
Another tip- If you remove the laminate on the black side of the heat tape it is about 10X easier. The Silver side has the laminate adhesive on it and is a real pain to thoroughly remove from the buss bar. Give the wire a good firm tug after the solder has cooled, a "cold" solder joint will give way very easily. We like the 15 watt or 40 watt iron from Radio Shack- cheap and easy to come by. Don't use the same iron you use to melt holes in plastic tubs, the plastic residue can sometimes make it hard to get a good joint.
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