Most hobbiests seem to name soldering as their number one area which could use improvement. Below are some suggestions which will help to get a nice smooth solder bead for copper foil.
The Right Tools/ Chemicals
Jennifer prefers the Weller 1175 Soldering Iron in combination with Glastar's Temperature Control for most copper foil soldering.
This is the most versatile iron we've found. The tapered chisel tip allows for getting into tight spaces as in 3-D work (lead angels, boxes), and you can cover large areas as well (4-way lead joints), since the useful area of the tip is around 3/4".
A hotter iron is preferred for soldering lampshades, brass, or copper. We have a great (and inexpensive) 150 watt iron for these.
For regular foil soldering, we like Denver flux. It's a "high speed, professional flux", which means it's easier to solder a smooth bead. It has zinc chloride, which makes your solder flow better than other fluxes.
Flux cleans and prepares the metal surface to be soldered. Weaker fluxes must be reapplied often if you are going back over previously soldered areas. Denver flux does not usually need to be reapplied.
For 3-D projects like boxes, Moravian Stars and Sun Jewels, we use Laco flux (non zinc-chloride). It is easier to clean off.
Always ventillate your soldering area well. We like to use a fan to blow solder fumes past us, and one to exhaust them out the window or door.
Wear safety glasses to protect against solder splatters, and if you have cuts on your hands, bandage them up or put on gloves.
Do not eat, drink or smoke in the immediate soldering area, and keep solder and lead away from children and pets.
Flux your project well. It will help your solder to flow properly. Use as much heat as you can handle, as a too-cool iron will really slow you down and cause your solder to form peaks. Jennifer mostly solders on "full" on the control, even when using a 150 watt iron.
If you are constantly getting "melt throughs", then either you are moving too slowly, holding your iron in one place too long, or your iron is too hot for you. Turn the temperature setting down.
Try to hold your iron at about a 45 degree angle to the table. If you hold it too flat, the solder will not raise up into a nice bead. The rounded bead looks best, and gives strength to the project.
Move as quickly as the heat will allow. In order to get a smooth, rounded bead, you need to coordinate all these things:
Angle of the iron
Amount of solder you feed into the tip
This is where you get all the sputtering and popping. It's not really from too much flux! What is happening is that, since the first side is sealed (soldered), until the solder from this side fills in all the spaces, you are making little "volcanoes".
The intense heat of the molten solder mixed with air and liquid is just like a mini Mount St. Helen. Until the solder totally fills in, the air and flux heats up and pops through the solder, causing it to pit.
You can overcome this by first flat soldering the entire second side. Once it's all filled in, and no more sputtering is happening, you can begin your beading.
It doesn't do away with all the popping, but it's faster and less annoying to work out the bubbles with a flat seam first.
It really helps to watch someone work who is excellent at soldering. There are other things we do for smooth seams which are hard to understand without seeing them demonstrated.